SQL TutorialWhat is SQL?

SQL stands for Structured Query Language SQL is pronounced as /ˌɛs.kjuːˈɛl/ SQL is designed for manipulate data in relational database management system

What can you do with SQL? SQL can retrieve and update data from relational database management system by

using data manipulation language. SQL can create and modify database schema and database objects such as tables,

views, indexes... via data definition language. SQL can grant or revoke authorization to user through data control language.

SQL Data Manipulation Language (DML)

SQL data manipulation language (DML) is a main part of SQL statements. SQL DML allows you to retrieve data from the database tables in relational database management system (RDBMS). In addition, SQL DML allows you change or delete data from the database tables by using UPDATE and DELETE statements. Here are all the SQL statements related to the SQL data manipulation language:

1. SQL SELECT2. SQL WHERE

3. SQL ALIAS

4. SQL DISTINCT

5. SQL ORDER BY

6. SQL IN

7. SQL BETWEEN

8. SQL LIKE

9. SQL GROUP BY

10. SQL HAVING

11. SQL INNER JOIN

12. SQL OUTER JOIN

13. SQL SELF-JOIN

14. SQL SUBQUERIES

15. SQL UNION

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16. SQL INSERT

17. SQL UPDATE

18. SQL DELETE

SQL Data Definition Language (DDL)

SQL Data definition language is a subset of SQL statements. SQL Database definition language allows yout to create, alter or remove different kind database objects such as tables, views, indexes.

1. SQL CREATE TABLE

2. SQL ALTER TABLE

3. SQL DROP TABLE

SQL Tutorial References(for Practical)

SQL Sample DatabaseWe use Northwind database as the sample database through all SQL tutorials in our website. The Northwind sample database provides you a good database structure and sample data to allow you to experiment with SQL statements you learn in each tutorial. We also try to make the tutorial as common as possible so you can practice with various Relational Database Management Systems (RDBMS). The sample database is also available in almost common RDBMSs such as MS Access, MS SQL Server, MySQL. Here is the database diagram of the Northwind sample database:

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SQL SELECTSummary: In this tutorial, you will learn how to use SQL SELECT statement to retrieve data from a database table.

The SQL SELECT statement allows you to retrieve the data from one or more database table. Here is the common syntax of the SQL SELECT statement:

1 SELECT column_list2 FROM table_list3 WHERE row_conditions4 GROUP BY column_list5 HAVING group_conditions6 ORDER BY sort_list ASC | DESC

First you have to specify the table name that you want to retrieve the data. The table name is followed by the keyword FROM. Second you have to indicate what data in want to retrieve. You can list the specify the column of the database table after the keyword

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SELECT. Suppose you want to retrieve last name of employees in the employees table, you can perform the following query:

1 SELECT lastname2 FROM employees

The query fetches the values from the column lastname in all rows in the employees table and returns the result below.

You can also use SQL SELECT statement to retrieve multiple columns from a database table. In order to do so you must provide a comma between columns in the column list. For example, if you want to retrieve first name, last name and title of all employees, you can execute the following query:

1 SELECT lastname, firstname, title2 FROM employees

To retrieve all information of employee in the employees table, you can list all columns name and separate them by commas. In case a database table having many columns and you do not want to list them all after the keyword SELECT, you can use an asterisk (*) to indicate that you want to retrieve all columns without explicitly specifying them in the column list. For instance, the following query allows you to retrieve all employee's information by using the asterisk (*):

1 SELECT *2 FROM employees

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In this tutorial, you’ve learned how to use simple SQL SELECT statement to retrieve data from a  table. You’ve also learned how to select a single column, multiple columns by separating them with a comma, and all columns by using the asterisk (*) from a table.

SQL WHERESummary: In this tutorial, you will learn how to use SQL WHERE clause with SQL SELECT statement to filter the data you select from database tables.

The SQL WHERE clause is used with other SQL statements such as SQL SELECT, SQL DELETE and SQL UPDATE statements to filter records in the database tables which satisfy  specific row conditions. SQL provides you various operators to allow you to construct row conditions. Here are most common operators in SQL.

Operator Description= Equal>  Greater than<  Less than>= Greater than or equal<= Less than or equal<>  Not equalAND Logical operator ANDOR Logical operator OR

Suppose you want to find all employees who has last name is King, you can perform this query:

SELECT lastname, firstname, titleFROM employeesWHERE lastname = 'King'lastname firstname title -------- --------- --------------------King Robert Sales Representative

SQL first fetches all rows from employees table which specified in the FROM clause. Next SQL eliminates rows which do not satisfy the row condition, in this case rows which has lastname column is not equal to ‘King’. Then SQL eliminates all columns which are not available in the column list (or sometimes refered as selection list).

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To find all employees who do not live in US, you can use not equal operator (<>) in WHERE clause as follows:

SELECT lastname, firstname, title, countryFROM employeesWHERE country <> 'USA'lastname firstname title country--------- --------- -------------------- -------Buchanan Steven Sales Manager UK Suyama Michael Sales Representative UK King Robert Sales Representative UK Dodsworth Anne Sales Representative UK

To find all employees who were hired before 1993, you can use less than operator ( < ) like this:

SELECT lastname, firstname, title, country,date(hiredate)FROM employeesWHERE hiredate < '1993-01-01'lastname firstname title country date(hiredate)--------- --------- --------------------- ------- --------------Davolio Nancy Sales Representative USA 1992-05-01 Fuller Andrew Vice President, Sales USA 1992-08-14 Leverling Janet Sales Representative USA 1992-04-01

To find all employees who were hired after 1993, you just use the greater than operator ( > ) in WHERE clause.

SELECT lastname, firstname, title, country,date(hiredate)FROM employeesWHERE hiredate > '1993-01-01'lastname firstname title country date(hiredate)--------- --------- ------------------------ ------- --------------Peacock Margaret Sales Representative USA 1993-05-03 Buchanan Steven Sales Manager UK 1993-10-17 Suyama Michael Sales Representative UK 1993-10-17 King Robert Sales Representative UK 1994-01-02 Callahan Laura Inside Sales Coordinator USA 1994-03-05 Dodsworth Anne Sales Representative UK 1994-11-15

The logical operators such as OR and AND are used to combine multiple conditions in WHERE clause.

SQL WHERE with AND operator

Multiple conditions are joined by AND operator must be TRUE for entire condition evaluate to TRUE. If one of condition is false, the entire condition evaluates to FALSE as well. If one of condition is unknown or NULL the entire condition evaluates to UNKNOWN. (You will learn how to deal with NULL in the next tutorials later).

  TRUE FALSE UNKNOWNTRUE TRUE FALSE UNKNOWNFALSE FALSE FALSE FALSEUNKNOWN UNKNOWN FALSE UNKNOWN

Suppose you want to find all employees who were hired after 1993 and lived in UK, you can use AND operator to combine both conditions like the following query:

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SELECT lastname, firstname, title, country, date(hiredate)FROM employeesWHERE hiredate > '1993-01-01' AND country = 'USA'lastname firstname title country date(hiredate)-------- --------- ------------------------ ------- --------------Peacock Margaret Sales Representative USA 1993-05-03 Callahan Laura Inside Sales Coordinator USA 1994-03-05

SQL WHERE with OR operator

Multiple conditions joined by OR operator must be FALSE for entire condition is evaluated to FALSE. If one of condition is TRUE, the entire condition is evaluated to TRUE. If one of condition is UNKNOWN or NULL the entire condition is evaluated to UNKNOWN or NULL.

  TRUE FALSE UNKNOWNTRUE TRUE TRUE UNKNOWNFALSE TRUE FALSE UNKNOWNUNKNOWN TRUE UNKNOWN UNKNOWN

For example you can find all employees who live in London or Seattle city, you can use OR operator in this case.

SELECT firstname, lastname, cityFROM employeesWHERE city = 'London' OR city = 'Seattle'firstname lastname city --------- --------- -------Nancy Davolio SeattleSteven Buchanan London Michael Suyama London Robert King London Laura Callahan SeattleAnne Dodsworth London

In this tutorial, you’ve learned how to filter the records in the result set in the SQL SELECT statement by using the SQL WHERE clause with conditions. You’ve also learned how to use various common basic operators to construct a condition and use logical operators to combine condition together. In the next tutorial you will learn how to use other operator such as SQL   BETWEEN and SQL IN to retrieve data in a range of values and in a set.

SQL AliasIn this tutorial, you will learn how to use different SQL alias including column alias and table alias with SQL   SELECT statement .

SQL Alias is used to organize the output and avoid ambiguous table error in SQL statements when multiple tables with the same column names are refered to.

SQL supports two types of alias which are known as column alias and table alias.

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SQL Column Alias

Basically when you select data from a database table, the column headings of the output are the same as columns’ name. However, the column's name heading is too technical that does not bring you any information therefore you want to change it in the output. SQL provides you SQL alias to allow you to do so. You can assign a column alias in the column list of the SELECT statement by using AS keyword.

Let’s take a look at several examples of using SQL column alias.

1 SELECT productName AS product,2        unitPrice   AS price3 FROM products4 WHERE unitPrice >50product price ------------------------ --------Mishi Kobe Niku 97.0000Carnarvon Tigers 62.5000Sir Rodney's Marmalade 81.0000Thüringer Rostbratwurst 123.7900Côte de Blaye 263.5000Manjimup Dried Apples 53.0000Raclette Courdavault 55.0000

In the SELECT   statement above, we used two column aliases. The first column alias is product which represents for productname and the second alias is the price which represent for unitprice.

In case the column name is lengthy and sometimes it is designed as an abbreviation, the column alias enables the output more meaningful and easy to read.

Be noted that the AS keyword is optional. You can omit the AS keyword in the column alias. If the column alias contain space but it must be enclosed in quotes. We can rewrite the above SELECT   query as follows:

1 SELECT productName product,2        unitPrice "unit price"3 FROM products4 WHERE unitPrice > 50

SQL Table Alias

SQL table alias is another name which you put after the table name of FROM clause of SELECT statement followed after table's name. SQL table alias is used when you refer a table multiple times in a SELECT statement with JOIN clause or when the table name is too long that you do want to make it easier to read and maintain.

For example you can select organization structure to find out who is manager of whom by using SQL JOIN with table alias as follows:

1 SELECT E.lastname "Employee name",2        M.lastname "Manager name"3 FROM employees E4 INNER JOIN employees M ON M.employeeID = E.ReportsToEmployee name Manager name

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------------- ------------Davolio Fuller Leverling Fuller Peacock Fuller Buchanan Fuller Suyama Buchanan King Buchanan Callahan Fuller Dodsworth Buchanan

In above query, we refered to the same table employee.  In the FROM clause we use E for employee as table alias and in the INNER JOIN clause we M for manager as table alias. When you use table alias, the column has to be refered as follows to avoid ambiguous column name error.

1 table_alias.column_name

SQL alias is is very useful when using with SQL   subqueries , SQL self join and SQL   INNER   JOIN  statements.

In this tutorial, you've learned how to use SQL alias including column alias and table alias.

You use column alias to reorganize or reformat the output to make the output more meaningful.

You use table alias when you address a table multiple times in a single SQL SELECT statment to make it easier to write and maintain the lengthy query.

SQL DISTINCTIn this tutorial, you will learn how to use SQL DISTINCT with SQL SELECT statement to eliminate duplicate records.

SQL DISTINCT is used to eliminate the duplicated rows in the result of SELECT statement. For example, to retrieve all the cities which employees live you can use SELECT statement as follows:

1 SELECT city2 FROM employeescity --------Seattle Tacoma KirklandRedmond London London London Seattle London

As you see, you get the result with the duplicate row which you don’t expect. In this case, you can use SQL DISTINCT to eliminate all of duplicate city in the result set by performing the following SQL statement:

1 SELECT DISTINCT city

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2 FROM employeescity --------Seattle Tacoma KirklandRedmond London

You’ve put DISTINCT keyword before the column name which you want it distinction in value. You can even put more columns after the DISTINCT keyword. At this time, the combination of all columns is used to evaluate the rows are duplicate or not.

For instance, you want to know all cities and countries information which employees live in; you can answer this question by performing the following query:

1 SELECT DISTINCT city, country2 FROM employeescity country-------- -------Seattle USA Tacoma USA Kirkland USA Redmond USA London UK

At this time the combination of city and country is used to determine the uniqueness of record in the result set.

Beside DISTINCT keyword, you can use ALL keyword to indicate that you don’t want to eliminate duplicated records. Because ALL keyword is default to the SELECT statement so you don’t have to explicitly specify it in the SELECT statement.

SQL ORDER BYIn this tutorial, you will learn how to use SQL ORDER BY clause to sort the result set based on criteria.

SQL ORDER BY statement allows you to sort the result set based on one or more sort keys in ascending or descending fashion. Here is the syntax of SQL ORDER BY:

1 SELECT column1, column2…2 FROM table_name3 ORDER BY sort_key1 [ASC | DESC], sort_key2 [ASC | DESC] …

SQL ORDER BY can only be used in SQL SELECT statement. The sort_key (sort_key1, sort_key2,...) in SQL ORDER BY must be sortable. It can be character, numeric or date time data type.

With SQL ORDER BY you can specify the sort order by using keyword ASC (means sort in ascending order) and DESC (means sort in descending order). If you don’t specify the sort order by using ASC and DESC keywords, the default ordering is ascending. You can either sort one or more columns in any sort order you want. 

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Let's take a look at several examples of using SQL ORDER BY:

For example, you can sort all employees by their last name by performing the following query. In this case you sort last name in ascending order. 

1 SELECT lastname, firstname2 FROM employees3 ORDER BY lastnamelastname firstname--------- ---------Buchanan Steven Callahan Laura Davolio Nancy Dodsworth Anne Fuller Andrew King Robert Leverling Janet Peacock Margaret Suyama Michael

You can also sort the result based on last name in descending order and first name in ascending order by performing the following query. This is known as multi column sorting.

1 SELECT lastname, firstname2 FROM employees3 ORDER BY lastname DESC, firstname ASClastname firstname--------- ---------Suyama Michael Peacock Margaret Leverling Janet King Robert Fuller Andrew Dodsworth Anne Davolio Nancy Callahan Laura Buchanan Steven

SQL sorts the result based on the last name in descending order first. And based on this sorted result, it then sorts the first name in ascending order.

SQL ORDER BY can also accept any expression. For example, you can use CONCAT function, which allows you concatenate multiple strings in into one, to construct  full name of employees and then sort them all by full name. Here is the query example:

1 SELECT CONCAT(lastname,',',firstname) fullname2 FROM employees3 ORDER BY fullnamefullname ----------------Buchanan,Steven Callahan,Laura Davolio,Nancy Dodsworth,Anne Fuller,Andrew King,Robert Leverling,Janet Peacock,Margaret

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Suyama,Michael

Almost RDMBS allows you to specify the sort key based on the positional number of column in the selection list. The starting position of column in the selection list is 1 and so on… and those positional numbers can be listed in SQL ORDER BY clause.

Suppose you want to sort employees by hired date to find out who are the most new hire employees of the company; You can use positional number in ORDER BY clause as follows:

1 SELECT lastname, firstname, date(hiredate)2 FROM employees3 ORDER BY 3 DESClastname firstname date(hiredate)--------- --------- --------------Dodsworth Anne 1994-11-15 Callahan Laura 1994-03-05 King Robert 1994-01-02 Buchanan Steven 1993-10-17 Suyama Michael 1993-10-17 Peacock Margaret 1993-05-03 Fuller Andrew 1992-08-14 Davolio Nancy 1992-05-01 Leverling Janet 1992-04-01

SQL sorts the result by hiredate column which positional number of hiredate column in the selection list is 3.

Because the positional number is changed when you add more columns in the selection list so you have to change it also in the SQL ORDER BY clause. This sometimes led you to an unexpected result if you forget to change the positional number. Therefore it is not recommended to use positional number in ORDER BY clause, you only use it if you don't have any option.

In this tutorial, you’ve learnt how to use SQL ORDER BY clause to sort the result in ascending and descending order.

SQL INIn this tutorial, you will learn how to use SQL IN operator along with SQL WHERE clause to retrieve data in a set of values.

SQL IN operator allows you to determine if a value is contained in a set of values. The syntax of using SQL IN operator is as follows:

1 SELECT column_list2 FROM table_name3 WHERE column IN (value1, value2, value3…)

The set of values must be comma-delimited and enclosed within parentheses.

To find all products which have unit price are 18, 19 and 20 you perform the following query:

1 SELECT productName, unitPrice2 FROM products

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3 WHERE unitPrice IN (18, 19, 20)productName unitPrice---------------- ---------Chai 18.0000Chang 19.0000Steeleye Stout 18.0000Inlagd Sill 19.0000Chartreuse verte 18.0000Maxilaku 20.0000Lakkalikööri 18.0000

The query above can be rewritten as following by using OR operator to combine multiple conditions.

1 SELECT productName, unitPrice2 FROM products3 WHERE unitPrice = 18 OR unitPrice = 19 OR unitPrice = 20

As you can see, SQL IN operator helps you to reduce the complexity of combining multiple OR conditions therefore it make SQL statement easier to understand and maintain.

To find all records which has a column value are not in a set you can use NOT IN. for instance, you can find all products which have unit price not 18 and not 19 and not 20 by performing the following query:

1 SELECT productName, unitPrice2 FROM products3 WHERE unitPrice NOT IN (18, 19, 20)

Here is the excerpt of the result

productName unitPrice--------------------------------- ---------Aniseed Syrup 10.0000Chef Anton's Cajun Seasoning 22.0000Chef Anton's Gumbo Mix 21.3500Grandma's Boysenberry Spread 25.0000Uncle Bob's Organic Dried Pears 30.0000Northwoods Cranberry Sauce 40.0000Mishi Kobe Niku 97.0000Ikura 31.0000Queso Cabrales 21.0000Queso Manchego La Pastora 38.0000Konbu 6.0000Tofu 23.2500...

Beside these above usages, SQL IN operator is also used in subquery which you will learn later in SQL subquery tutorial.

In this tutorial, you’ve learn how to use SQL IN operator to find records which has value in a set. Furthermore, you’ve also learnt how to combine NOT operator with SQL IN operator to find all records which has value not in a set.

SQL BETWEEN

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In this tutorial, you will learn how to use SQL BETWEEN operator with SQL WHERE clause to select the records which have the value in a range of values.

SQL BETWEEN operator allows you to retrieve records which has value in a range of values.The syntax of SQL BETWEEN is as follows:

1 SELECT column_list2 FROM table_name3 WHERE column BETWEEN lower_value AND upper_value

Be noted that SQL BETWEEN operator gets all records which have column value is lower_value and upper_value.

For example you want to retrieve products which have unit price from 18$ to 19$, you can use SQL BETWEEN operator like the following query:

1 SELECT productName, unitPrice2 FROM products3 WHERE unitPrice BETWEEN 18 AND 19productName unitPrice---------------- ---------Chai 18.0000Chang 19.0000Steeleye Stout 18.0000Inlagd Sill 19.0000Chartreuse verte 18.0000Boston Crab Meat 18.4000Lakkalikööri 18.0000

You can also reconstruct SQL BETWEEN by using less than or equal and greater than or equal operators like the following query:

1 SELECT productName, unitPrice2 FROM products3 WHERE unitPrice >= 18 AND unitPrice <= 19

To find all products which does not have unit price between that ranges of prices, you can combine SQL NOT operator with SQL BETWEEN operator. Here is the query:

productName unitPrice------------------------------- ---------Mishi Kobe Niku 97.0000Konbu 6.0000Teatime Chocolate Biscuits 9.2000Sir Rodney's Marmalade 81.0000Tunnbröd 9.0000Guaraná Fantástica 4.5000Thüringer Rostbratwurst 123.7900Geitost 2.5000Côte de Blaye 263.5000Jack's New England Clam Chowder 9.6500Rogede sild 9.5000Zaanse koeken 9.5000Filo Mix 7.0000Tourtière 7.4500

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Rhönbräu Klosterbier 7.7500

In this tutorial you’ve learned how to use SQL BETWEEN operator to find values which are in a range of values. In addition you’ve also learned how to combine SQL NOT operator and SQL BETWEEN operator to find all records which have value is not in a range of values.

SQL LIKE StatementIn this tutorial, you will learn how to use SQL LIKE to select data based on pattern matching

SQL LIKE statement allows you to perform search string of text based on patterns. SQL LIKE statement is used in the WHERE clause of any valid SQL statements such as SELECT, INSERT, UPDATE and DELETE.

SQL provides you two wildcard characters to construct a pattern. They are percentage (%) and underscore (_).

1. Percentage (% ) wildcard allows you to match a sequence of any characters including space.

2. Underscore ( _ )  wildcard allows you to match any single character.

The syntax of SQL LIKE statement is  as follows:

1 SELECT column1, column2…2 FROM table_name3 WHERE column LIKE pattern

The data type of column must be alphanumeric in order to use SQL LIKE statement. It could be CHAR, VARCHAR, NVARCHAR… data type.

Let’s take a look at several examples of using SQL LIKE statement and constructing patterns.

Suppose you want to find all employees with the last name starting with D character, you can use perform the following query.

1 SELECT lastname, firstname2 FROM employees3 WHERE lastname LIKE 'D%'lastname firstname--------- ---------Davolio Nancy Dodsworth Anne

The expression ‘D%’ means find all string starting with character ‘D’ and followed by any characters.

To find all employees which have the first name ending with character ‘t’, you can execute the following query.

1 SELECT lastname, firstname 2 FROM employees

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3 WHERE firstname LIKE '%t'lastname firstname--------- ---------Leverling Janet Peacock Margaret King Robert

The expression ‘%t’ means any string with any characters in any length and ending with character ‘t’.

You can put the wildcard ‘%’ at the beginning and the end of a string to find any string which contains string within those wildcards. For example to find all employees which have last name contain string “ll”, you can execute the following query as follows:

1 SELECT lastname, firstname 2 FROM employees3 WHERE lastname LIKE '%ll%'lastname firstname-------- ---------Fuller Andrew Callahan Laura

Two wildcard characters ‘%’ and ‘_’ can combine together to construct a pattern. For example you can find all employees which have last name starting with any single characters, followed by character ‘a’ and followed by any characters. You can use the combination of both wildcard characters. Here is the query to do so:

1 SELECT lastname, firstname2 FROM employees3 WHERE lastname LIKE '_a%'

SQL LIKE statement can also combine with the SQL NOT operator to find all string which does not match the pattern. For example if you want to find all employees which first name is not starting with character ‘D’, you can perform the following query:

1 SELECT lastname,firstname 2 FROM employees3 WHERE lastname NOT LIKE 'D%'lastname firstname--------- ---------Fuller Andrew Leverling Janet Peacock Margaret Buchanan Steven Suyama Michael King Robert Callahan Laura

In this tutorial, you’ve learned how to use SQL LIKE statement to find string of text which matches a pattern. You’ve learned how to use two wildcard characters: percentage (%) and underscore (_) to construct a pattern for using in SQL LIKE statement.

SQL GROUP BY16

In this tutorial, you will learn how to SQL GROUP BY clause to group a set of result set based on group columns.

SQL GROUP BY is used to divide a database table into groups based on group columns. For each group you can apply aggregate functions such as SUM, AVG, MIN, MAX and COUNT to output the summary information. SQL GROUP BY is very useful when you want to analyze data in analytical way such as how many sale orders was ordered by a customer and sold by a sale person. The common syntax of SQL GROUP BY is as follows:

1 SELECT c1,c2,... cn, aggregate_function(expression)2 FROM tables3 WHERE where_conditions4 GROUP BY c1, c2, ... cn5 ORDER BY order_columns

Let’s take a look at several examples of using SQL GROUP BY to see how it works.

Suppose you want to find the total money of every sale order you have in order details table. You can use SQL GROUP BY and SUM function to do so. Here is the query:

1 SELECT orderID, SUM(unitPrice * quantity)2 FROM order_details3 GROUP BY orderID

Here is the excerpt of the result

orderID SUM(unitPrice * quantity)------- ------------------------- 10248 440.0000 10249 1863.4000 10250 1813.0000 10251 670.8000 10252 3730.0000 10253 1444.8000 10254 625.2000 10255 2490.5000 10256 517.8000 10257 1119.9000 10258 2018.6000 10259 100.8000 10260 1746.2000

SQL engine first looks at the GROUP BY clause and groups the table into groups based on the order identity. Then SQL engine computes the sum of unitPrice column multiple with quantity column for each group.

You can use SQL GROUP BY without the aggregate functions. At this time SQL GROUP BY acts like SELECT DISTINCT to distinguish all records based on group columns. Here is the query:

1 SELECT orderID, SUM(unitPrice * quantity)2 FROM order_details3 GROUP BY orderID

Here is the excerpt of the result

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orderID SUM(unitPrice * quantity)------- ------------------------- 10248 440.0000 10249 1863.4000 10250 1813.0000 10251 670.8000 10252 3730.0000 10253 1444.8000 10254 625.2000 10255 2490.5000 10256 517.8000 10257 1119.9000 10258 2018.6000 10259 100.8000 10260 1746.2000

You can use SQL GROUP BY clause together with SQL ORDER BY clause to sort the output result. For example you can sort the total sale of all sale orders in descending order as the following query:

1 SELECT OrderID, SUM(unitPrice * quantity) total2 FROM Order_details3 GROUP BY OrderID4 ORDER BY total DESC

Here is the excerpt of the result

OrderID total ------- ---------- 10865 17250.0000 11030 16321.9000 10981 15810.0000 10372 12281.2000 10424 11493.2000 10817 11490.7000 10889 11380.0000 10417 11283.2000 10897 10835.2400 10353 10741.6000

SQL GROUP BY with multiple columns

You can use SQL GROUP by with multiple columns in GROUP BY clause. For example, if you want to know how many sale orders were booked by a customer and sold by a employee, you can count sale order number and group by customer and employee. Here is the SQL statement to do so:

1 SELECT b.customerid,  a.employeeid ,2        COUNT(a.orderid) AS cnt3 FROM orders a4 INNER JOIN customers b ON a.customerid = b.customerid5  6 GROUP BY b.customerid7         ,a.employeeid8 ORDER BY b.customerid, 9          cnt DESC

Here is the excerpt returned data from the query above:

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+------------+------------+-----+| customerid | employeeid | cnt |+------------+------------+-----+| ALFKI | 4 | 2 || ALFKI | 1 | 2 || ALFKI | 6 | 1 || ALFKI | 3 | 1 || ANATR | 3 | 2 || ANATR | 7 | 1 || ANATR | 4 | 1 || ANTON | 3 | 3 || ANTON | 7 | 2 || ANTON | 4 | 1 || ANTON | 1 | 1 || AROUT | 4 | 4 || AROUT | 1 | 3 || AROUT | 3 | 2 || AROUT | 9 | 2 |+------------+------------+-----+

In this tutorial, you’ve learned how to use SQL GROUP BY to divide records in a database table into groups and apply aggregate function for each group to produce summarization output. In the next tutorial, you will learn HAVING clause which is related to GROUP BY, let’s move on.

SQL HavingSQL HAVING clause is used together with SQL GROUP BY clause to filter group of records based on conditions. SQL HAVING clause is similar to WHERE clause in functionality manner but filter group of records not records.

 » SQL Tutorial » SQL HAVING

SQL HavingSQL HAVING clause is used together with SQL GROUP BY clause to filter group of records based on conditions. SQL HAVING clause is similar to WHERE clause in functionality manner but filter group of records not records.

Let’s take a look at several examples of using SQL HAVING.

If you want to find all sale orders which have total sale greater than $120000, you can use HAVING together with the GROUP BY clause.

1 SELECT OrderID, SUM(unitPrice * quantity) total2 FROM Order_details3 GROUP BY OrderID4 HAVING total > 12000OrderID total ------- ---------- 10372 12281.2000 10865 17250.0000 10981 15810.0000 11030 16321.9000

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Suppose you want to find all orders which have a number of products greater than 5 , you can use COUNT function with HAVING and GROUP BY together. Her e is the query:

1 SELECT orderID, COUNT(productID) prd_count2 FROM order_details3 GROUP BY orderID4 HAVING prd_count > 5orderID prd_count------- --------- 10657 6 10847 6 10979 6 11077 25

SQL INNER JOINSQL JOIN allows you to combine records from two or more tables into a temporary table called join-table. SQL provides five types of join: inner join, outer join, right join, left join and self-join.

Sample tables

We have two tables: categories and products in the sample database to demonstrate how the SQL JOIN works. Here is the database diagram of them:

 

1. One category can have multiple products.2. One product belongs to only one category.

SQL INNER JOIN

The syntax of SQL INNER JOIN is as follows:

1 SELECT selection_list2 FROM table_A 3 INNER JOIN table_B ON join_condition4 WHERE row_conditions.

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Table_A and table_B  are sometimes called joined-tables. SQL INNER JOIN returns records from both tables where a match is found based on join conditions (join_condition). SQL INNER JOIN gets all the records from the table_A and finds the matching records in the table_B according to the join condition. The join condition determines whether both records are matched or not. If there is no match found, no records is returned. Suppose you need the following information: productId, productName, and category name of each product from both tables products and categories. In this case, you can use SQL INNER JOIN to combine the records in both tables as follows: 1 SELECT productID,productName,categoryName2 FROM products3 INNER JOIN categories ON products.categoryID = categories.categoryID

Here is the excerpt output result from the query above:

productID productName categoryName --------- --------------------------------- -------------- 1 Chai Beverages 2 Chang Beverages 3 Aniseed Syrup Condiments 4 Chef Anton's Cajun Seasoning Condiments 5 Chef Anton's Gumbo Mix Condiments 6 Grandma's Boysenberry Spread Condiments 7 Uncle Bob's Organic Dried Pears Produce 8 Northwoods Cranberry Sauce Condiments 9 Mishi Kobe Niku Meat/Poultry 10 Ikura Seafood ....

For each product in the products table, SQL finds  a corresponding category in the categories table which has the same categoryID; If there is a match found, SQL returns records otherwise no record is returned. Most of the time you use foreign keys to form the join condition. In this case categoryID is the foreign key of two tables. 

There is another form of SQL INNER JOIN which called implicit inner join. Here is the implicit SQL INNER JOIN syntax:

1 SELECT selection_list2 FROM table_A, table_B3 WHERE join_condition.

In this form you list all joined-tables after the FROM clause and put join condition in WHERE clause of the SQL SELECT statement.  As our above example, we can rewrite the example query as follows: 

1 SELECT productID,productName,categoryName2 FROM products, categories3 WHERE products.categoryID = categories.categoryID

There is another way to visualize the SQL INNER JOIN by using the Venn diagrams. In this way you can see that SQL INNER JOIN returns only records that match in both table_A and table_ B.

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In this tutorial, you've learned how to use SQL INNER JOIN to retrieve data  from two tables. In the next tutorial, you will learn how to use another kind of SQL join called outer join.

SQL OUTER JOINDifferent from the SQL INNER JOIN, SQL OUTER JOIN returns all records from both joined tables even there is no matching record found. There are three types of SQL OUTER JOIN: FULL OUTER JOIN, LEFT OUTER JOIN and RIGHT OUTER.

SQL LEFT OUTER JOIN

Suppose we join two tables A and B. Left outer join returns all records from the table A (left table) plus matching records in the table B (right table).  It means the result of the SQL LEFT OUTER JOIN always contains the records in the table A (table in the left side) even no matching record found in the table B plus matching records in the table B.

Here is the Venn diagram to visualize how SQL LEFT OUTER JOIN works.

 

The syntax of left outer join is as follows:

1 SELECT * FROM table_A2 LEFT OUTER JOIN table_B ON join_conditions3 WHERE row_conditions

SQL RIGHT OUTER JOIN

SQL RIGHT OUTER JOIN returns all records from the table B (table in the right side), even no matching record found in the table A, plus matching records in the table A.

Venn diagram to visualize how SQL RIGHT OUTER JOIN works:

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The syntax of SQL RIGHT OUTER JOIN is as follows:

1 SELECT column_list 2 FROM table_A3 RIGHT OUTER JOIN table_B ON join_conditions4 WHERE row_conditions

SQL FULL OUTER JOIN

SQL FULL OUTER JOIN combines results of both left outer join and right outer join therefore it returns all records from both tables.

Venn diagram to visualize how SQL Full Outer Join works:

 

The syntax of SQL Full outer join is as follows:

1 SELECT column_list 2 FROM table_A 3 FULL OUTER JOIN table_B ON join_conditions4 WHERE row_conditions

In this tutorial, you’ve learned how SQL OUTER JOIN works and all kinds of SQL OUTER JOIN: SQL LEFT OUTER JOIN, SQL RIGHT OUTER JOIN and SQL FULL OUTER JOIN.

SQL Self-joinSQL self-join simply is a normal join which is used to join a table to itself. The SQL self-join can be done by using SQL table aliases to treat one table like a different table and then join them together. SQL self-join can be any form of join such as SQL   inner join , SQL   outer join… so you can apply any join to the SQL self-join.

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Here is common syntax of SQL self-join:

SELECT column_list FROM table_A AS AINNER JOIN table_A AS B ON A.column_name1 = B.column_name2, ...WHERE row_conditions

SQL self-join is very useful when you want to retrieve related data storing in one table such as organizational structure. In our database sample, we have employees table which stores not only employee data but also organizational structure. The column reportsTo specifies the manager of  an employee and is referenced to employeeID column. 

To list all information of employees and managers we can use SQL self-join as follows.

SELECT concat(e.firstname, e.lastname) employee, concat(m.firstname,m.lastname) managerFROM employees eINNER JOIN employees m ON m.employeeId = e.reportsTo

Here is the output of the sql self-join query above:

employee manager-------- -------Nancy Andrew Janet Andrew Margaret Andrew Steven Andrew Michael Steven Robert Steven Laura Andrew

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Anne Steven

SQL SubqueriesSQL subquery is a SQL query which is nested in another query. A SQL subquery is usually nested inside a SELECT, INSERT, UPDATE and DELETE statement. A SQL subquery can be used anywhere in a query where an expression is allowed. A SQL subquery is somtimes refered as inner select or inner query. The SQL statement contains a subquery is also called outer select or outer query.

Here is an example of using an SQL subquery as an expression in SELECT statement as MaxDiscount column:

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1 SELECT orderid, 2        orderdate, 3     (SELECT MAX(discount) 4      FROM order_details 5      WHERE orderid = orderid) AS maxdiscount6 FROM orders

A SQL subquery nested in an outer SELECT statement has several components as follows:

A  selected field in a regular SELECT statement to use as a data source for the outer SELECT statement.

A regular FROM clause to specify the table where a selected field is chose from. Optional clauses such as WHERE, GROUP BY or HAVING. 

A SELECT statement of a SQL subquery must be enclosed in parenthesises.

A SQL subquery can include one or more subquery. A number of subquery can include in a subquery depends on the implementation of each RDBMS. It is not recommended to have a high number of leve of nesting subquery in a SQL statement. The following SQL query is used to find the total quantity product bought by customers from USA.

01 SELECT o.productid, p.productname, SUM(o.quantity) quantity02 FROM order_details o03 INNER JOIN products p ON p.productid = o.productid04 WHERE o.orderid IN ( SELECT orderid FROM orders05             WHERE customerid IN (SELECT customerid 06                     FROM customers 07                     WHERE country = 'USA' ) )   08                      09 GROUP BY productid  10 ORDER BY quantity DESC     

The inner most query returns all the customer number from USA. The query at the next higher level is evaluated with these customer numbers and returns all sale orders numbers from those customers. Finally outer query uses the order numbers to find all product number from order details table. Product name is selected by join order details table with product tables. The total of quantity is calculated by using aggregate function SUM and GROUP BY clause.

Subqueries Examples

Subqueries with Aliases

In SQL statements if the subquery and outer query refers to the same table. In this case we have to use SQL Alias for both outer query and subquery. The following query finds all managers of the company by refering table employees twice in outer query and subquery.

1 SELECT e1.employeeid, e1.firstname, e1.lastname2 FROM employees e13 WHERE e1.employeeid IN (4     SELECT e2.reportsto5     FROM employees e2 )

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Subqueries with Comparison Operators

A subquery can be used with comparison operators such as (=, < >, >, > =, <, ! >, ! <, or < =). For example, we can use subquery with comparison operator to find all products which has prices greater than average prices as follows:

1 SELECT productName2 FROM products3 WHERE unitprice > (SELECT AVG(unitprice) 4            FROM products)

Subqueries with UPDATE, DELETE and INSERT statements.

A subquery can nested in the DML statements such as UPDATE, DELETE and INSERT statement. Here is a subquery to update the unit price of all products supplied by vendor number 15.

1 UPDATE products2 SET unitprice = unitprice * 0.53 WHERE productid IN (SELECT productid 4             FROM suppliers5             WHERE supplierid = 15)

Subqueries as expression

A subquery can be used as an expression in anywhere where expression is allowed in the SQL statement. Here is an example of using subquery as an expression to list all product average price and difference between product's price and the average price in product category 1 (Beverages).

1 SELECT productid, productname,2   (SELECT AVG(unitprice) FROM products) average,3    unitprice - (SELECT AVG(unitprice) FROM products)4 FROM products5 WHERE categoryid = 1

Correlated subquery

Correlated subquery is a special kind of subquery which the subquery uses the values from outerquery in its where clause. The subquery is evaluated for every rows selected by the outerquery.

The following query is a typical example of correlated query for finding products which have price greater than average unit price in its category.

1 SELECT productid, productname,unitprice2 FROM products p13 WHERE unitprice > (SELECT AVG(unitprice) 4                    FROM products 5                    WHERE categoryid = p1.categoryid)

SQL UNION

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SQL UNION is used to combine results of two or more SQL SELECT queries into one. The syntax of SQL UNION is as follows:

1 SELECT column_list1 FROM table12 UNION (DISTINCT | ALL)3 SELECT column_list2 FROM table2

Basically the first and the second query can be any SQL SELECT queries with a restriction that the column_list1 and column_list2 have to be compatible. It means both columns lists must have the same number of columns, and each corresponding column must has the same data type or at least convertible data type.

By default SQL UNION eliminates all duplicate records, in this case NULL values are considered as a single value. To enable duplication of records, you can use ALL keyword followed after UNION explicitly. Be noted that by default the DISTINCT is used if you don’t specify anything after UNION.

Let’s take a look at several examples of using SQL UNION. Suppose you want to find all the cities of all customers and suppliers. In this case, you can use SQL UNION to combine cities of both customers and suppliers as follows: 

1 SELECT city FROM customers2 UNION3 SELECT city FROM suppliers

Here is the excerpt output:

city ---------------Aachen Albuquerque Anchorage Barcelona Barquisimeto Bergamo Berlin Bern Boise Brandenburg Bruxelles

If you use UNION ALL you will see duplicate values in the output by performing the following query:

1 SELECT city FROM customers2 UNION ALL3 SELECT city FROM suppliers

In this tutorial, you've learned how to use SQL UNION to combine result of two or more SQL SELECT queries into one.

SQL INSERT

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SQL INSERT statement allows you to insert one or more records into a database table. In addition SQL INSERT statement also allows you to copy data from a database tables to another database table.

SQL INSERT statement is sometimes referred as SQL INSERT INTO statement.

Insert one record into a database table.

Here is the syntax of SQL INSERT statement which allows you to insert one record at a time into a database table.

1 INSERT INTO table_name (column1, column2…)2 VALUES (value1, value2,…).

In this form, you specify the name of database table, which you want to insert data into, followed after the INSERT INTO statement. The number of columns (column1, column2,...) and values (value1, value2,..) must be same. If the column is omitted, the default value for column is used. The values inserting to the database table must also satisfy other conditions such as constraints of foreign key, NOT null constraints… otherwise the insert action will be failed and new row is not added to the database table.

Let’s take a look at an example of inserting data into the Shippers table by using SQL INSERT statement.

1 INSERT INTO Shippers (companyName, phone)2 VALUES ('Alliance  Shippers','1-800-222-0451')

Because shipperID is the primary key of the Shippers table and it is automatically increased each time we insert a new row so we don’t need to list it there in the column list. Only data we have to provide for INSERT statement  are company name and phone. After executing the query, the database server returns number of row affected. Here you get 1 row affect to indicate that one row has been added successfully/

Insert multiple records into a database table

SQL INSERT statement also allows you to insert multiple records into a database table at a time. Here is the syntax:

1 INSERT INTO table_name(column1,column2…)2 VALUES (value1,value2,…),3        (value1,value2,…),4 …

In this form, you provide multiple values which are corresponding to the column list of the database table. Here is an example of inserting multiple records into Shippers table.

1 INSERT INTO shippers(companyName,phone)2 VALUES ('UPS','1-800-782-7892'),3        ('DHL','1-800-225-5345')

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Copy data from another database table

Sometime you need to copy data from one table into another table for backing up for example. SQL INSERT statement  allows you to copy data from another database tables to a database table. Here is the syntax:

1 INSERT INTO table_name(column1, column2,…)2 SELECT value1, value2…3 FROM table_name24 …

The selection list must be corresponding to columns of the database table you want to copy data.

Suppose you have a temporary table called table_tmp with the structure exactly the same as the shippers table. Now you want to copy data from the shippers table into this temporary table, you can use SQL INSERT INTO SELECT statement. In this case by performing the following query:

1 INSERT INTO shippers_tmp (companyName, phone)2 SELECT companyName, phone3 FROM shippers

In this tutorial, you’ve learned how to use SQL INSERT statement to insert one or more records into a database table. In addition, you also learned how to copy the data from a database table to another database table by using SQL INSERT SELECT INTO statement.

SQL UPDATESQL UPDATE statement allows you to modify data in a database table. With SQL UPDATE statement, you can modify data of the whole table or subset of data based on condition in WHERE clause.

Here is the typical syntax of SQL UPDATE statement:

1 UPDATE table_name2 SET column1 = value1,3     column2  = value24 WHERE condition

First you specify a database table where you want to update data after UPDATE keyword. You can update data in one column or more. If you update data in more than one column, the columns has to be separated by commas (,).

It is not required to provide the data directly to the SQL UPDATE statement. You can also use data which you retrieve from another table and then use it for SQL UDPATE statement as well. The data you can get from a SELECT statement but be sure that the select query must return a single record which has type compatible with the column you want to update.

The WHERE clause is optional. If you omit the WHERE clause, all the records of the database table will be updated.

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Let’s take a look at several examples of using SQL UPDATE.

Suppose one of employee in the company get married and need to change her last name, so you have to make the change by using the SQL UPDATE statement. Here is the query to do so:

1 UPDATE employees2 SET lastname = 'Phan'3 WHERE employeeID = 3.

Suppose her employee ID is 3.

Another example is one of employee of the company change the address so you want to update address information including address, city, region and postal code. In this case you can use SQL UPDATE to change this  data.

1 UPDATE employees2 SET address = '1300 Carter St',3 city = 'San Jose',4 postalcode =  95125,5 region = 'CA'6 WHERE employeeID = 3

In this tutorial, you've learned how to use SQL UPDATE statement to update data in a database table.

SQL DELETEIn this tutorial, you will learn how to use SQL DELETE statement to remove data from database tables.

SQL DELETE statement allows you to delete one or more records in a database table. The syntax of SQL DELETE statement is as follows:

1 DELETE FROM table_name2 WHERE conditionsIf you omit the WHERE clause the SQL DELETE statement, it will delete all records in the database table. It is very time consuming and less efficient to use SQL DELETE statement to do so especially with the table with a big data. If you want to delete all data in a database table, SQL provide you TRUNCATE statement which is more efficiently to delete the whole table./div>

Let’s take a look at couples of examples using SQL DELETE statement.

If you want to remove employee number 3 just execute the following query:

1 DELETE FROM employees2 WHERE employeeID = 3

If the record which has employeeID with 3 exist, it will be deleted otherwise nothing happens.

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To delete all employees in the table employees (not recommended, and make a backup before you do this) you just execute the following query:

1 DELETE FROM employees

SQL DELETE statement become complicated when you delete a record in a table which has relationship and link together by a foreign key such as employees and employeeterritories. If you want to delete an employee you have to delete a record which has employeeid in employeeterritories table also. So you have to execute two DELETE statements as follows:

1 DELETE FROM employees2 WHERE employeeID = 3;1 DELETE FROM employeeterritories2 WHERE employeeID = 3

Almost RDBMS allows you to create a constraint called referential integrity between two linked tables. Therefore if a record in a table is deleted, other records in the linked table, if exist, are deleted also. Therefore in this case you just only have to execute the first DELETE query to make the data to be integrity.

In this tutorial, you’ve learned how to use SQL DELETE statement to delete one or more records in a database table. You’ve also learned about referential integrity constraints between tables to allow you to delete records in linked table automatically by deleting record in other table.

SQL CREATE TABLEIn this tutorial, you will learn how to use SQL CREATE TABLE statement to create database table.

Database table is a basic element of the database. Before storing data in a table you have to create it. SQL provides CREATE TABLE statement to allow you to do so. Here is the common syntax of SQL CREATE TABLE statement:

1 CREATE TABLE table_name(2 Column_name1 data_type(data_length ),3 Column_name2 data_type(data_length ),4 ..5 )

There are four components you need to specify when creating a database table:

1. Database table name. It is suggested that the name should be meaningful and in plural form of a noun. For example products is the name of a table which stores product data.

2. Name of each column in the table: It should be meaningful also and in a noun with singular form.

3. Data type for each column: Choose the correct data type for column is very important in the design phrase. The most common data types are text (varchar, nvarchar), numerical (smallint, int, bigint), date, time datetime, timespan,blob…you can refer it in the specific RDBMS.

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4. Maximum length of each column: You have to specify the maximum length of data for each column. For example if you store product name, try to imagine the maximum length a product can have, such as 255 characters.

Beside those components above you can have another additions for each column such as the column is mandatory or not (NULL or not NULL), default value of the column and value of the column is unique or not. If a column is unique and not null it can be a primary key of the table.

Here is an example of creating products table:

01 CREATE TABLE products (                                 02             ProductID int(11) NOT NULL AUTO_INCREMENT,            03             ProductName varchar(40) NOT NULL,                     04             SupplierID int(11) DEFAULT NULL,                      05             CategoryID int(11) DEFAULT NULL,                      06             QuantityPerUnit varchar(20) DEFAULT NULL,             07             UnitPrice decimal(19,4) DEFAULT NULL,                 08             UnitsInStock smallint(6) DEFAULT NULL,                09             UnitsOnOrder smallint(6) DEFAULT NULL,                10             ReorderLevel smallint(6) DEFAULT NULL,                11             Discontinued tinyint(4) NOT NULL,                     12             PRIMARY KEY (ProductID)13           )

In the above example, we've created a products table. The first column is ProductID which is the primary key of the product table which is used to differentiate between data rows or records. PRIMARY KEY keyword is used to specify the primary key of the table. There are also other columns used to describe a product of the products table with different data type, length and default value.

Once table is created, you can alter it by using SQL   ALTER   TABLE  or removing it using SQL   DROP   TABLE .

SQL ALTER TABLE

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 In this tutorial, you will learn how to use SQL ALTER TABLE to modify database table schema.

You can not only create and remove a database table but also modify an existing database table. SQL provides you ALTER TABLE statement to allow you to do so. With SQL ALTER TABLE statement you can do the following things on the database table:

Add a new column or removing an existing column of the table. Modify of maximum length of a column. Add or remove default value of a column. Add or remove constraints of table.

To add new column in a table you use the following syntax:

ALTER TABLE table_nameADD COLUMN new_column datatype(datalength)

First you specify the table you want to add a new column and then you specify column name, its data type and its maximum length of data. For example to add new column called availableOnline in the products table you can perform the following query:

ALTER TABLE ProductsADD COLUMN availableOnline BIT NOT NULL

To remove an existing column in a database table you need to specify the table and column name you want to remove. Here is the syntax:

ALTER TABLE table_nameDROP COLUMN existing_column

To drop the column availableOnline in products table we created in the above example we can use ALTER TABLE … DROP COLUMN to do so. Here is the query:

ALTER TABLE productsDROP COLUMN availableOnline

When you drop a column, all the data in that column is removed. If you drop a key column which is a part of a composite key of the table, not only the data in the column is removed but also all the duplicated records in that table are removed also.

The SQL ALTER TABLE statement can be different between database products in its additions such as add or remove constraints, set default value… you can check it out to know more in details.

In this tutorial, you’ve learnt how to modifying table by using SQL ALTER TABLE statement. You’ve learnt how to add or removing a column from a database table.

SQL DROP TABLEIn this tutorial, you will learn how to use SQL DROP TABLE statement to completely delete database table structure and data from a database.

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SQL DROP TABLE is used to remove the whole database table in term of table structure and data. When you use SQL DROP TABLE, the database engine removes all objects related to that table including data, table definition, indexes, table constraints, trigger and permission spec of that table. The syntax of SQL DROP TABLE is as follows:

1 DROP TABLE table_name

In order to remove table, you'll need to have sufficient permission to do so. In the statment you specify the table name you want to remove, the database will remove the table in the current working database. If you want to remove table in a different database you need to use the following syntax:

1 DROP TABLE database_name.table_name

In the above syntax, you specify the database followed by a full stop and table name.

SQL DROP TABLE examples

Here is an example of removing table employees in the current working database:

1 DROP TABLE employees

To remove table employees in the database tmp, you'll need to execute the following query:

1 DROP TABLE tmp.employees

Remarks on SQL DROP TABLE Before executing the SQL   DROP   TABLE  you need to to consider it seriously. You

have to asked yourself whether the table being removed is still used in the future. If this is the case, you have to backup the database first before droping table.

You should always find all the foreign key constraints or referencing table that reference to the table being removed. Then you remove all the references first before dropping table.

It is safer to specify the database name when you use SQL   DROP   TABLE  statement. Because in the database landscape you may have different database systems and names for development, testing and production systems. Sometimes you have to go to those systems at the same time to do the work. You may confuse the production, testing and development systems and potentially drop table in the production system with the thought that it is development system.

If you just want to delete the data use SQL   DELETE or SQL TRUNCATE statement instead.

SQL Cheat SheetThe SQL cheat sheet is designed to provide a quick reference for the most common SQL statements you use. It is one-page A4 printable document in both PDF and JPEG format. You can print it out and stick it to your desk.

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SQL Statements Cheat Sheet

SQL statements cheat sheet gives you the most common usage SQL query. You can find the most basic SQL statements from basic SQL statement to advanced SQL Join. In addition you can find the SQL statements for creating database objects such as tables, views and indexes.

MySQL Tutorial MySQL tutorial site provides you useful tutorials of SQL and MySQL.

SQL on Wikipedia SQL information on Wikipedia with definition of SQL, SQL history and other languages elements

PostgreSQL Guides PostgreSQL is one of the most advanced open source database management system. Postgresqlguide.com gives you basic guides about the PostgreSQL from the start.

SQL Tutorial Yet another SQL tutorial website, it mainly focus on SQL standard with practical examples. http://www.coderecipes.net/

SQL Data Definition CREATE TABLE Statement

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Table is a basic element of relational database and create table is very important before you are working with data which you want to store.

ALTER TABLE Statement

with SQL ALTER TABLE statement you can add, drop one or more columns from a table.

DROP Table Statement

DROP TABLE statement allows you drop an existing table in database which is useful in some programming situations.

How to Create SQL View

SQL View is a virtual table which is used to encapsulate a complex queries.Click here to see how to create SQL view

SQL ALTER VIEW Statement

SQL view is a virual table so it can be changed using ALTER VIEW statement.

SQL DROP VIEW Statement

SQL DROP VIEW statement removes view definition from the system catalog and the dependent objects of that view will become invalid.

Creating Database Index

A database index is similar to the book's index which helps you to find a topic faster by looking up on it.

SQL DROP INDEX statement

SQL DROP INDEX statement frees allocated memory and removes the index definition from the database information schema

Advanced SQL  SQL UNION Tutorial

SQL union allows you to combine the match result sets of two (or more than two) select queries into a single table.

SQL CASE Expression Tutorial

SQL CASE expression is used as a kind of IF-THEN-ELSE statement.

SQL MINUS or EXCEPT Operator

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SQL MINUS or EXCEPT operator work on two table expressions. The result set takes records from the first table expression, and then subtract out the ones that appear in the second table expression. If the second table expression includes the records which are not appear in the first table expression, these records will be ignored.

SQL INTERSECT Opertor Tutorial

INTERSECT operator allows you to combine two table expressions into one and return a result set which consists of rows that appear in the results of both table expressions.

SQL UNION OperatorSQL UNION operator is one of the basic set operations in the relational database. SQL UNION operator allows you to combine the match result sets of two (or more than two) select queries into a single table. Two result sets of the select queries must be made up of same kind elements such as same number of column, and same data type (or automatically cast to each other) in all columns. In most implementations, the data type conversion will be occurred but it is very dependent-implementations. You can check it out your RDBMS to find out. The duplicated records of two result sets are eliminate except theUNION ALL is used.

The common syntax of SQL union is very simple as follows:

query1UNION (ALL)query2

As an example here two tables which we will use to to demonstrate union operator.

cost2006 table

CostCenterId Cost ------------ ------ 1 300000 2 150000 3 500000

cost2007 table

CostCenterId Cost ------------ ------ 1 120000 2 250000 3 450000

This is the query to union two table data

SELECT * , 2006 AS yearFROM cost2007UNIONSELECT * ,2007 AS yearFROM cost2006

Here is the result which is sorted by cost centers:

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CostCenterId Cost year ------------ ------ ------ 1 120000 2006 1 300000 2007 2 250000 2006 2 150000 2007 3 450000 2006 3 500000 2007

In a good database model one table has one and only one type of elements therefore SQL UNION does not make sense here. It is usually used in a data warehouse application in which the database models are not good nomalized.

SQL CASE ExpressionA special scalar expression in SQL language is CASE expression. SQL CASE expression is used as a kind of IF-THEN-ELSE statement. It is similar to switch statement in modern programming language such as Java or C#. The syntax of the CASE statement is simple as follows :

1 CASE column_name2   WHEN condition1 THEN result13   WHEN condition2 THEN result24   ...5   ELSE result6 END

The data type of the column_name after the CASE must be the same as the data type of the expression followed by the keyword THEN or ELSE. The ELSE part of the case expression is optional. If the ELSE part is omitted and all the conditions in the WHEN does not meet, the CASE expression will return NULL.

The case expression can be used in anywhere scalar expressions are allowed, including in WHERE and HAVING clause of the select statement.

It is more intuitive to demonstrate CASE expression through an example. Here is the employees table for demonstration :

1 employee_id  name      department_id  job_id  salary 2 -----------  --------  -------------  ------  -------3           3  newcomer         (NULL)       0  2000.004           2  mary                  2       2  2500.005           5  Tom                   2       2  2700.006           4  anna                  1       1  2800.007           1  jack                  1       1  3000.008           6  foo                   3       3  4700.00

We can use the CASE expression to print out the employee name, his salary and a computed column which is called salary level. Here is the sql query:

1 SELECT name,salary,2 CASE   3 WHEN  salary <= 2000 THEN 'low'4 WHEN  salary > 2000 AND salary <= 3000 THEN 'average'

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5 WHEN  salary > 3000 THEN 'high'6 END AS salary_level7 FROM employees8 ORDER BY salary ASC

The logic is simple if the salary of the employee lower than 2000 the salary level is low; greater than 2000 and less than or equal to 300, the salary level is average; And greater than 3000, the salary level is high (of course just for example). And the output is :

1 name       salary  salary_level2 --------  -------  ------------3 newcomer  2000.00  low         4 mary      2500.00  average     5 Tom       2700.00  average     6 anna      2800.00  average     7 jack      3000.00  average     8 foo       4700.00  high  

SQL MINUS or EXCEPT OperatorSQL MINUS or EXCEPT operator work on two table expressions. The result set takes records from the first table expression, and then subtract out the ones that appear in the second table expression. If the second table expression includes the records which are not appear in the first table expression, these records will be ignored.

The syntax of using SQL MINUS is simple as follows :

1 table_expression12 MINUS3 table_expression21 table_expression12 EXCEPT3 table_expression2

Be noted that the MINUS operator only returns distinct values from table expression.

SQL MINUS or EXCEPT operator is one of the set operators. SQL MINUS or EXCEPT is equivalent of the difference operator from set theory. Some RDBMS uses MINUS and the other uses EXCEPT keyword.

SQL INTERSECT OperatorINTERSECT operator allows you to combine two table expressions into one and return a result set which consists of rows that appear in the results of both table expressions. INTERSECT operator, like UNION operator, removes all duplicated row from the result sets. Unlike the UNION operator, INTERSECT operator operates as AND operator on tables expression. It means data row appears in both table expression will be combined in the result set while UNION operator operates as OR operator (data row appear in one table expression or both will be combined into the result set). The syntax of using INTERSECT operator is like UNION as follows:

1 table_expression1

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2 INTERSECT3 table_expression2

Table expression can be any select statement which has to be union compatible and ORDER BY can be specified only behind the last table expression.

Be note that several RDBMS, including MySQL (version < 5.x), does not support INTERSECT operator.

This is an example of using INTERSECT operator. Here is the employees sample table

1 employee_id  name      department_id  job_id  salary 2 -----------  --------  -------------  ------  -------3           1  jack                  1       1  3000.004           2  mary                  2       2  2500.005           3  newcomer         (NULL)       0  2000.006           4  anna                  1       1  2800.007           5  Tom                   2       2  2700.008           6  foo                   3       3  4700.00

We can find employee who work in department id 1 and two and have salary greater than 2500$ by using INTERSECT operator. This example using the sample table in both table expressions, you can test it on two different tables.)

1 SELECT * 2 FROM employees3 WHERE department_id in (1,2)4 INTERSECT5 SELECT * 6 FROM employees7 WHERE salary > 25001 employee_id  name    department_id  job_id  salary 2 -----------  ------  -------------  ------  -------3           1  jack                1       1  3000.004           4  anna                1       1  2800.005           5  Tom                 2       2  2700.00

SQL Stored Procedure Tutorial Introducing to Stored Procedure

Stored procedure by definition is a segment of code which contains declarative or procedural SQL statement.

Getting Started with Stored Procedure

You will learn how to write the first stored procedure and how to call it.

Parameter List in Stored Procedure

A stored procedure can have zero, one or more than one parameters. If a stored procedure has more than one parameter, each one must be separated by a comma.

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Body Part of a Stored Procedure

The body part of a stored procedure is where you can put your business logic code inside to execute it as you want it to do.

Local Variables in Stored Procedure

Local variables are used in stored procedure to keep temporary intermediate results.

Common Usage Flow-Control in Stored Procedure

There are two most common usage of flow-control which are conditional execution and loop.

Modifying and Compiling Stored Procecdure

Once a store procedure is resided in the database server catalog, we can modify it by using the ALTER PROCEDURE statement or remove it by using DROP PROCEDURE

Introducing to Stored ProcedureStored procedure by definition is a segment of code which contains declarative or procedural SQL statements. A stored procedure is resided in the catalog of the database server so we can call it from a trigger, another stored procedure or even from client appliations.

As the definition above, the stored procedure can contains any SQL statement like INSERT, UPDATE and DELETE or any SQL data definition like CREATE TABLE, ALTER TABLE and etc. In addition, a stored procedure also supports procedure statements such as IF ELSE, WHILE... to make it as powerful as another programming languages such as C/C++, C# and Java.

Using stored procedure has several advantages :

It is used to increases the performance of application because when we create stored procedures, they are compiled and stored in database catalog. Later when client applications call them, they are generally executed faster than uncompiled SQL statements which are sent from the client applications.

The network traffic between application server and database server is also signification reduced because the applications don't have to send such long and uncompiled SQL statements to the server to get the data back.

Stored procedures can be used for database security purpose because each store procedure can have its own database privileges.

One of the most advantage of stored procedure is code reusability. Once created, a stored procedure can be reused over and over again by multiple applications.

It is the best to illustrate the ability of stored procedure by showing examples. You can follow this tutorial to understand more about stored procedure. We will use Microsoft SQL Server to demonstrate stored procedure, you can also use MySQL with a change a little bit because of specification of each database server is different. Start learning how to write a stored procedure by following the tutorial getting started with stored procedure

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Getting Started with Stored ProcedureWriting the first stored procedure

Here is the first stored procedure source code:

1 CREATE PROCEDURE Delete_Employee2    (@EmployeeId INT)3 AS4  5 BEGIN

6    DELETE FROM  Employees7    WHERE  EmployeeId = @EmployeeId;8 END

A stored procedure must contains at least three parts: stored procedure name, parameter list and its body.

The CREATE PROCEDURE is similar to CREATE TABLE or INDEX statement. It is actually a SQL statement. The CREATE PROCEDURE will force the database server add the stored procedure to the its catalog. The name of stored procedure is followed after the CREATE PROCEDURE statement, in this case it is Delete_Employee. It would be the best that the name is meaningful and follows by the naming convention of the database server specification, for example each stored procedure should begin with "sp". In almost relation database product, the name of stored procedure must be unique.

The second part of the stored procedure is parameter list. In this case the list contains only one parameter @EmployeeId (the employee identity). Microsoft SQL Server requires prefix @ for every parameters and variables of stored procedure. Followed each parameter is its type, in this case, its type is integer (INT).

The main part of a stored procedure is the stored procedure body. It starts with keywords BEGIN and ends with keyword END. In this example the body is very simple; It deletes employee by employee identity.

When all syntax statements inside body are correct, the database server will store the stored procedure name and code in its catalog for reusing later by another stored procedure or programs.

Calling a stored procedure

We can call a stored procedure from the console window, from another stored procedure or from a program which can access database server. The syntax of calling a stored procedure is simple as follows:

1 EXEC spName(parameter_value_list)

The EXEC statement is used to invoke a stored procedure. After the EXEC statement is the stored procedure name followed by parameter list. This is an example to delete an employees with identity is 8 by calling the sample procedure above Delete_Employee:

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1 EXEC Delete_Employee(8)

If a stored procedure has more than one parameters, the values of them can be passed to it and separated by a comma.

As you see writing and calling a stored procedure is very simple and easy. In the following tutorials, we will show you the feature and syntax of a stored procedure along with statement which can be used inside the body so you can empower its power. Next you will learn how to use parameter list to pass and get data in and out of stored procedures parameter list in stored procedure.

Parameter List in Stored ProcedureA stored procedure can have zero, one or more than one parameters. If a stored procedure has more than one parameter, each one must be separated by a comma.

A parameter can be described by three parts : name, data type and its type which can by IN, OUT or INOUT.  The first two pars are mandatory and the third part is optional.

The name of parameter in Microsoft SQL Server must has @ sign as the prefix  Otherwise the database server will notify the error and of course the stored procedure cannot be saved in the database catalog.

Following the name is the parameter data type. The data type of the stored procedure can be any valid data type which are predefined in the database server. If you specify the size of the data type you can do it as follows:

1 @parameter VARCHAR(255)

The third part of a parameter is its types. This part is optional, by default it is the IN. There are three parameter types: IN, OUT and INOUT. As you guess IN is abbreviation of input, OUT is abbreviation of output and INOUT is combined of both. With input parameter you can pass the value to the stored procedure. As in the previous tutorial example, we pass the employee identity to the storprocedure and delete the employee based on it. Output parameter allows you get the value back from the stored procedure, for example you may write a stored procedure to insert an employee and get its id back to the application to use in another part of the program. The INOUT parameter type as its name suggested can act as both types of parameter.

In some SQL database sever product, it does not require parameter has @ sign as prefix so you should be careful that the name of the parameter must not be equal to the name of column otherwise you may face up to the disaster without any warning or error message from the database server.

In Microsoft SQL Server 2005/20008, it allows you to specify the default value of the parameter. this is a big plus because you can call a stored procedure without passing parameters, It will use default values. Here is syntax of default value of parameter.

1 @parameter DataType(Size) = Default_Value

In our example we can modify to use default value for stored procedure as follows:

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1 CREATE PROCEDURE Delete_Employee2    (@EmployeeId INT = 0)3 AS4  5 BEGIN

6    DELETE FROM  Employees7    WHERE  EmployeeId = @EmployeeId;8 END

When you call it without passing parameter value, it will delete the employee with the identity is zero.

SQL stored procedure parameter list is simple and easy to grasp? let's move to the body part of the stored procedure.

Body in Stored ProcedureSponsored Links

The body part of a stored procedure is where you can put your business logic codesinside to execute it as you want it to do. A stored procedure body always begin with BEGIN and END keywords. Inside the body, you can put the declarative SQL statements or procedure call like calling other stored procedures. Beside that, you can also use procedure statement like IF, WHILE; you can declare local variables and use them inside the stored procedure. Here is the general stored procedure syntax :

<create procedure statement> ::= CREATE PROCEDURE <procedure name> ( [ <parameter list> ] ) AS <procedure body>

<procedure body> ::= <begin-end block>

<begin-end block> ::= [ <label> : ] BEGIN <statement list> END [ <label> ]

<statement list> ::= { <body statement> ; }...

<statement in body::= <declarative statement> | <procedural statement>

<declarative statement> ::= <EXEC statement> | <CLOSE statement> | <COMMIT statement> | <DELETE statement> | <EXECUTE immediate statement> | <FETCH statement> | <INSERT statement> | <LOCK TABLE statement> | <OPEN statement> | <ROLLBACK statement> | <savepoint statement> | <SELECT statement> | <SELECT INTO statement> |

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<SET statement> | <SET transaction statement> | <start -transaction statement> | <UPDATE statement>

<procedural statement> ::= <BEGIN-END block> | <EXEC statement> | <CLOSE statement> | <DELCARE condition statement> | <DELCARE cursor statement> | <DELCARE handler statement> | <DELCARE variable statement> | <FETCH cursor statement> | <flow control statement> | <OPEN cursor statement> | <SET statement>

With the BEGIN and END keword you can label the block of code inside the body. You can have one or more blocks, each block can be nested each other. Labeling the block has its own advantages. For example, it makes your code more clean when you have mutilple blocks. Let's get your hand with some source code to demonstrate the stored procedure body.

Imagine we have employee table, in one day the table may have many records and it is very costly to get all the data from it to display them in our application. It would be nice if we can provide pagination feature for application to select needed records it needs to reduce the traffic between the database server and application server. Here is stored procedure to make it possible:

CREATE PROCEDURE GetEmployeePaged @PageSize int = 10,-- pagesize

@CurrentPage int = 1,-- current page no@ItemCount int output -- total employee found

ASBEGIN

-- declare local variables for paginationDECLARE @UpperBand INT,

@LowerBand INT SET @LowerBand = (@CurrentPage - 1)* @PageSize

SET @UpperBand = @CurrentPage* @PageSize + 1

-- assign itemcount output parameterSET @ItemCount = (

SELECT COUNT(employeeId) FROM employees

)

-- create temporary table to store paged dataCREATE TABLE #ALLROW(

RowID INT PRIMAY KEY IDENTITY(1,1),EmployeeId INT,Name VARCHAR(255),salary DECIMAL(7,2)

)

-- insert data into the temporary tableINSERT INTO #ALLROWSELECT EmployeeId, Name, salaryFROM employees

-- get paged data SELECT *

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FROM #ALLROWWHERE RowID > @LowerBand AND RowID < @UpperBand

END

First in parameter list we have three parameters and their meanings are exactly what they are. @pagesize specifies number of record per page, @currentpage specifies the current page number and @itemcount specifies total record found. So we can get the employee record in the page 1 with 10 record per page by calling:

1 EXEC GetEmployeePaged(10,1,@itemcount)

The next we declare two local variables. These variables are used inside the stored procedure for determining the start row and end row we will retrive the records. These variables' values are calculated based on the @pagesize and @currentpage.

Then we use SET statement to assign output parameter @itemcount to the total records of the employee.

Finally we create a temporary table to store the data, insert the data into the temporary table and retrieve the needed records.

As you can see the stored procedure is very flexible, you can leverage it to deal with tough situation in database developement.

In the next tutorial we will show you how to use local variable and use SET statement as shown in the example above in the stored procedure. Next tutorial: Local variables in stored procedure

Local Variables in Stored ProceduresLocal variables are used in stored procedure to keep temporary intermediate results. In order to use a local variable we have to declare it explicitly as follows:

1 <declare variable statement> ::=2    DECLARE <variable list> <data type> [3       DEFAULT <expression> ]4  5 <variable list> ::=6    <variable> [ { , <variable> }... ]

DECLARE keywords is used to declare local variables. This source code snippet shows you how to declare a numeric and an alphanumeric variables:

1 DECLARE @Found INT2 DECLARE @Firstname VARCHAR(255)

Some database server support initial value for local variables and some not. If supported, we can specify the default value for local variable. The expression for the default value of local variables are not limited to literals (like 'name' or 1) but may consist of compound expressions, including scalar subqueries.

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Local variable has its own scope. It does not exist when the stored procedure finish. Inside each code block local variable only visible it the block it declared.

The SET statement can be used to assign value to local variable. Here is the syntax :

1 <set statement> ::=2    SET <local variable definition>3        [ {, <local variable definition> }... ]4  5 <local variable definition> ::=6    <local variable> { = | := } <scalar expression>

We can use SET statement to assign value or expression to local variable. You can refer to the example in stored procedure body tutorial.

Local variable is intermedia storage to store temporary result in stored procedure.In the next tutorial, we will show you how to use flow-control statements in stored procedure.

Common Usage Flow-Control in Stored ProcedureIn this section, we will cover all basic flow-control statements which can be used in stored procedures. There are two most common usage of flow-control which are conditional execution and loop.

Conditional execution with IF-THEN-ELSE and CASE statement

The IF-THEN-ELSE statement is used to evaluate the value of boolean expression; if the value is True it execute the block code that follows it otherwise it will execute the block code that follows by ELSE.The ELSE part is optional in thestatement.

Here is an stored procedure which finds the maximum value between two integers using IF-THEN-ELSE statement. The example is easy just for demonstration.

01 CREATE PROCEDURE FindMax02     @v1 INT,03     @v2 INT,04     @m INT OUTPUT05 AS06 BEGIN07     IF @v1 > @v208         SET @m = @v109     ELSE10         SET @m = @v211 END

In complex cases, we can use CASE statement instead of IF-THEN-ELSE statement. CASE statement evaluates a list of boolean expression and returns one of multiple possible result expressions. CASE statement is similar to the swith-case statement in other programming languages such as C/C++, C# or Java. Here is an example of using CASE statement to display salary level of employee. We have employee table data as follows:

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1 employee_id  name      salary 2 -----------  --------  -------3           1  jack      3000.004           2  mary      2500.005           3  newcomer  2000.006           4  anna      2800.007           5  Tom       2700.008           6  foo       4700.00

And here is the stored procedure example:

01 CREATE PROCEDURE DisplaySalaryLevel02 AS03 BEGIN

04     SELECT  employeeId, name,salary, salary_level =05     CASE salary06         WHEN salary < 1000 07             THEN 'very low'08         WHEN salary > 1000 AND salary < 2000 09             THEN 'low'10         WHEN salary > 2000 AND salary < 4000 11             THEN 'average'12         WHEN salary > 4000 AND salary < 10000 13             THEN 'high'14         WHEN salary > 10000 15             THEN 'very high'16     END17     FROM Production.Product18 END

Looping with WHILE statement

Since T-SQL is fourth generation language and designed to operate with sets of data therefore it is also possible to write the code to loop through the record set and perform operations on a single record. Using loop will cause the performance of the server but in some cases it is necessary to use this feature. Here we will you an example of using WHILE loop statement to calculate the factorial of an integer number.

 

01 CREATE PROCEDURE Cal_Factorial02   @inyN INT,03   @intFactorial BIGINT OUTPUT04 AS

05 BEGIN  06     SET @intFactorial = 107  08     WHILE @inyN > 109     BEGIN10         SET @intFactorial = @intFactorial * @inyN11         SET @inyN = @inyN - 112     END13 END

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In the real world database programming you will need to use the conditional statements to write own your stored procedures. Next we will show you how to modify stored procedure using ALTER and DROP statement

Modifying and Compiling Stored ProcecdureOnce a stored procedure is resided in the database server catalog, we can modify it by using the ALTER PROCEDURE statement as follows:

1 ALTER PROCEDURE spName(parameter_list)2 AS3 BEGIN4     -- stored procedure body here5 END

To remove a stored procedure from database catalog, we can use DROP PROCEDURE.

1 DROP PROCEDURE spName

When you remove stored procedure, be noted that some database products have a feature that allows you to remove the dependency database objects like table, view, index or other stored procedures also.

Stored procedure is compiled before it executes. Each database server has its own compiling strategy. You can specify the compiling option when you with WITH RECOMPILE statement as follows:

1 CREATE PROCEDURE spName2    (parameter_list) AS3    WITH RECOMPILE4 BEGIN5     -- stored procedure body here6 END

The WITH RECOMPILE statement guarantees that each time the stored procedure is invoked, the compiler is called and compile the stored procedure again. With WITH RECOMPILER the stored procedure is recompiled and adjusted to the current situation of the database which brings some advantages to the processing strategy. But be noted that the compilation process takes time and also decreases the performance of the database server. Therefore for each stored procedure ,we can specify which is the best to use WITH RECOMPILE.

SQL Functions SQL Aggregate Functions

Aggregate SQL functions summarize the results of an expression for the group of rows in database table and returns a single value for that group.

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SQL Mathematical Functions

SQL mathematical functions are very important to all programming languages and also to SQL. Here we cover all common SQL mathematical functions such as ABS, POWER, SQRT, RANDOM...with clear explanations and source code examples

SQL String Functions

SQL string functions allow you to manipulate character data effectively.

 » SQL Functions » SQL Aggregate Functions

SQL Aggregate Functions SQL MAX and MIN function

The SQL MAX() and MIN() functions allow you to display maximum and minimum values in a set.

SQL COUNT Function

The SQL COUNT() function used to count number of record of each database table.

SQL AVG Function

The SQL function AVG() calculates the arithmetic average of the series of numbers of its argument

SQL SUM Function

The SQL SUM() function uses to sum the values

SQL MAX and MIN FunctionsThe SQL MAX() and MIN() functions allow you to display maximum and minimum values in a set.

1 MAX ([DISTINCT]|[ALL] <expression>)2 MIN([DISTINCT]|[ALL] <expression>)

Here we have employee table:

1 employee_id  name      salary 2 -----------  --------  -------3           1  jack      3000.004           2  mary      2500.005           3  newcomer  2000.006           4  anna      2800.007           5  Tom       2700.008           6  foo       4700.00

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To find the highest and lowest salary of employees in the list, the following query could be used:

1 SELECT MAX(salary) max_salary,2 MIN(salary) min_salary3 FROM employees

Here is the output:

1 max_salary  min_salary2 ----------  ----------3    4700.00     2000.00

SQL COUNT FunctionThe SQL COUNT function is used to count a number of records of each database table. Here is the syntax:

1 COUNT([DISTINCT]|[ALL] <expression>)

Let's take a look at employees table

1 employee_id  name      salary 2 -----------  --------  -------3           1  jack      3000.004           2  mary      2500.005           3  newcomer  2000.006           4  anna      2800.007           5  Tom       2700.008           6  foo       4700.00

This query counts a number of employees of the employee table:

1 SELECT COUNT(*) as total_employee2 FROM employees

Here is the output:

1 total_employee2 --------------3              6

SQL AVG FunctionThe SQL AVG function calculates the arithmetic average of the series of numbers of its argument. The syntax is simple as follows:

1 AVG ([DISTINCT]|[ALL] <numeric expression="">)

For example, the following query could be used to calculate the average salary of all employees in employees table:

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1 SELECT AVG(salary) average_salary  2 FROM employees1 average_salary2 --------------3    2950.000000

SQL MAX and MIN FunctionsThe SQL SUM() function uses to sum the values. Here is the syntax:

1 SUM([DISTINCT]|[ALL] <numeric_expression>)

We have the sample table: Employee

1 employee_id  name      salary 2 -----------  --------  -------3           1  jack      3000.004           2  mary      2500.005           3  newcomer  2000.006           4  anna      2800.007           5  Tom       2700.008           6  foo       4700.00

In order to calculate the total amount of salary the company has to pay each month for all employees, we can use this query:

1 SELECT SUM(salary) AS total_salary2 FROM employee

Here is the output:

1 total_salary2 ------------3     17700.00

SQL Mathematical Functions SQL CEIL() (or CEILING) and FLOOR() Functions

Both SQL CEIL(or CEILING) and FLOOR functions are round the numbers. SQL CEIL roundup to the nearest integer value while FLOOR round down to the next least integer value.

SQL RAND Function

The SQL RAND() function is used to generate some random numbers at run time.

SQL ABS Function

SQL ABS function returns the absolute value of an numeric input argument.

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SQL EXP Function

The SQL EXP() function returns e raised to the n-th power (e is the base of the natural logarithm and is equal to approximately 2.718281828459).

SQL PI Function

SQL PI() function maybe the most simplest mathematical SQL function because it returns a constant value of pi

SQL SQRT Function

The SQL SQRT function extracts the square root from the positive numeric input argument. SQL SQRT function accepts only positive arguments, and returns an error when a negative argument is passed in.

SQL POWER Function

The SQL POWER() function returns the

SQL ROUND Function

The SQL ROUND function rounds a number to a specific length or precision.

SQL LOG Function

The SQL LOG() function allows us to operate mathematical logarithmic calculation

SQL Trigonometric Functions

SQL trigonometric functions allow you to operates trigonometric calculations.Here are some common SQL trigonometric functions you need to know in your daily SQL programming.

Database Trigger  Introducing to Database Trigger

Trigger is a kind of stored procedure means it can contains declarative or procedural SQL statements except one thing it cannot be invoked explicitly by user or other stored procedures.

Triggers are a kind of stored procedure which are invoked automatically by database server when predefined events occurred. Events here can be altering the tables, drop the tables, adding or removing rows on a table. Trigger is a kind of stored procedure means it can contains declarative or procedural SQL statements except one thing it cannot be invoked

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explicitly by user or other stored procedures. It only can be invoked when a predefined event occurred. Because of this feature, triggers are usually consider as events in the database.

Triggers can be applied in some contexts :

Triggers can helps database administrator gather statistic data on table access. Triggers can be used to provide transparent logging service to log data changes and

then send notification to the database administrator. Triggers can be used to generate and maintain derived data in derived column such as

computed column. Triggers can be used to enforce data integrity when the business rule of data is too

complex and prevent invalid transaction. And triggers can be used to enforce complex security authorization context..

Triggers are useful for use in the database but it is suggested that it should be used only when necessary. The triggers can put the burden of interdependency on the database and also burden of maintenance.

Triggers was added to SQL3 standard but different database servers implement trigger in different ways. So it is easier to demonstrate trigger in a specified database server. We will use Microsoft SQL Server 2005 for demonstration We surely also try to provide you resources where you can find how other database servers implements trigger.

Database Cursor Introducing to Database Cursor

In some database management contexts, we need to traverse through a set or records, process each record, move to next record and loop until the final record is reached. This is the reason why database cursors has invented.

Using Transact-SQL Cursor Syntax

In this section, you will learn to use required steps to work with cursor.

Introducing to Database CursorSQL was designed to work with a set of data and SELECT statement is the most important statement. SQL statement allows us to select a record or a set of records based on some criteria and it solves almost problem in database management. But in some database management contexts, we need to traverse through a set or records, process each record, move to next record and loop until the final record is reached. This is the reason why database cursors has invented.

There are many types of cursors are implemented in various RDBMS products using different syntax. Even though, the way we work with cursors are following this fashion:

1. We have to define a cursor with and set its features2. Then we must populate a set of records to use with the cursor

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3. We must set the position for cursor to a record or set of records that we need to traverse through

4. Data from one or more current records is fetched, then we can make some modification on it.

5. Loop the step 3 and 4 until the cursor reaches the final record in the set of record6. Finally we have to close and release the cursor resources.

In this tutorial, we will use Transact-SQL cursors for demonstration. We also provide you resources if you work with other RDBMS such as MySQL, Oracle... Transact-SQL cursor can be used from stored procedure, functions or triggers. Let's move to the section which we will show you how to use Transact-SQL cursors.

Using Transact-SQL Cursor SyntaxIn order to using Transact-SQL cursors, you have to perform the following steps:

Create a cursor with declare cursor statement based on a SELECT statement. 

1 DECLARE cursor_name CURSOR2 FOR select_statement

Populate the cursor with Open statement.

1 OPEN {{[GLOBAL] cursor_name} cursor_variable_name} 2     

Use the fetch statement to change the current record in the cursor and stores that record into local variables

1 FETCH  [[NEXT | PRIOR | FIRST | LAST2               | ABSOLUTE {n | @NVAR}3               | RELATIVE {n | @NVAR}4        ]FROM]5  {{[GLOBAL] cursor_name }|@cursor_variable_name}6  [INTO @variable_name[, . . .n] ]

Process the retrieved data Loop the step 3 and 4 until the final record in the result set is reached Close the cursor with close statement

1 CLOSE {{[GLOBAL] cursor_name }|cursor_variable_name}2     

Deallocate the cursor by using Deallocate statement.

1 DEALLOCATE {{[GLOBAL] cursor_name }|@cursor_variable_name}

It would be easier to explain how to use transact-SQL cursor work via an example. Let's move to the an example how to use cursor.

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