Difference between char, varchar, nchar and nvarchar data type in SQL Server databaseWhat is difference betweencharandvarcharin SQL, followed byncharandnvarchar, is one of the popular SQL interview question, and surprisingly not every programmer knows this basic difference. If you go with name, which you should, than you can figure out thatcharis a fixed length data type whilevarcharshould be variable length data type. Though allchar,varchar,nchar, andnvarcharare used to store text orString datathere are subtle differences between them. As I said char is fixed length, which means a variable or a column e.g.Zipcode char(10)will take only 10 bytes to store data, including space. On the other hand avarcharvariable or column will take variable space, depending upon data you store + 2 additional bytes for storing length. For example a varchar column namevarchar name(20)will take 6 bytes if you store"Jack"(4 + 2)and 7 bytes if you store "Jones" (5 + 2). In order to get better performance, you should use char for fixed length columns e.g.zipcode, where every row are under certain length e.g. 6 for India, and 5 + 4 digits postal codes for USA. On the other hand, for a variable length column it's better to use varchar data type to save the space, which is lost in case of char type, if actual data is always way less than capacity. In particular this question is next in series of couple of popular SQL interview question, e.g.difference between WHERE and HAVING clauseand writingSQL query to join three tables. If you come across any other interesting SQL queries than you can also share with us, if you don't know answer, we will try to find out together.

NCHAR vs NVARCHAR in SQLNow let's see difference betweenncharandnvarchardata types in SQL.nchar(n)andnvarchar(n)are similar to there counterpart char and varchar, but they take twice space than them, to support multilingual languages. Additional space is used to store Unicode character for eachcharacter, which means 2 bytes for each character, 1 for character + 1 for Unicode. Apart from this fundamental question, you often see couple of follow-up questions likewhen to use char and varchar in database?or more frequentlywhat is difference between char(20) and varchar(20) variables, as they can only store 20 bytes to store data. Well, main difference is that varchar might take less bytes, depending upon length of data you store on it. For example if we store"USA"in a variable of typechar(20)andvarchar(20)than first will take 20 bytes, while second will take only 5 bytes (3 +2 ), but performance of char columns are better inSELECT querythan varchar columns. Similarly if we usenchar(20)andnvarchar(20)for storing"SQL"string than first will take 40 bytes(2*20)and second variable will take 8 bytes (3*2 +2).

Here is a summary of how much space achar(10),varchar(10),nchar(10)andnvarchar(10)variable takes for storing same data :declare@cDatachar(10)set@cData ='SQL'-- 10 bytesset@cData ='Java'-- 10 bytes

declare@vcDatavarchar(10)set@vcData ='SQL'--3 + 2 = 5 bytesset@vcData ='Java'-- 4 + 2 = 6 bytes

declare@ncDatanchar(10)set@ncData ='SQL'-- 10*2 = 20 bytesset@ncData ='Java'-- 10*2 = 20 bytes

declare@nvcDatavarchar(10)set@nvcData ='SQL'-- 3*2+2 = 8 bytesset@nvcData ='Java'-- 4*2+2 = 10 bytes

Thanks guys, that's all ondifference between char and varchar data types in SQLandnchar vs nvarchar.I hope this knowledge of basic, not only helps in SQL interviews but also choosing right type for your columns in tables and variables in your stored procedures. Let me know if you have any doubt or question on char or varchar variables.

THURSDAY, AUGUST 1, 2013Difference between WHERE vs HAVING clause in SQL - GROUP BY Comparison with ExampleWhat is difference betweenWHEREandHAVINGclause in SQL is one of the most popular question asked on SQL and database interviews, especially to beginners. Since programming jobs, required more than one skill, its quite common to see couple ofSQL Interview questionsinJava and .NET interviews. By the way unlike any other question, not many Java programmers or dot net developers, who is supposed to have knowledge of basic SQL, fail to answer this question. Though almost half of the programmer says thatWHEREis used in anySELECTquery, whileHAVINGclause is only used in SELECT queries, which contains aggregate function or group by clause, which is correct. Though bothWHEREandHAVINGclause is used to specify filtering condition in SQL, there is subtle difference between them. Real twist comes into interview, when they are asked to explain result of aSELECT query, which contains bothWHEREandHAVINGclause, I have seen many people getting confused there. Key point, which is alsomain difference betweenWHEREandHAVINGclause in SQLis that, condition specified inWHEREclause is used while fetching data (rows) from table, and data which doesn't pass the condition will not be fetched into result set, on the other handHAVINGclause is later used to filter summarized data or grouped data. In short if bothWHEREandHAVINGclause is used in aSELECT query with aggregate functionorGROUP BYclause, it will execute beforeHAVINGclause. This will be more clear, when we will see an example ofWHERE,HAVING,JOINandGROUP BYclause together.

WHERE vs HAVING Clause Example in SQLIn this example ofWHEREandHAVINGclause, we have two tablesEmployeeandDepartment.Employeecontains details of employees e.g. id, name, age, salary and department id, whileDepartmentcontains id and department name. In order to show, which employee works for which department we needtojoin two tablesonDEPT_IDto get the the department name. Our requirement is to find how many employees are working in each department and average salary of department. In order to useWHEREclause, we will only include employees who are earning more than 5000. Before executing our query which containsWHERE,HAVING, andGROUP BYclause, let see data fromEmployeeandDepartmenttable:

SELECT*FROMEmployee;EMP_IDEMP_NAMEEMP_AGEEMP_SALARYDEPT_ID

1Virat23100001

2Rohit2470002

3Suresh2580003

4Shikhar2760001

5Vijay2850002

SELECT*FROMDepartment;DEPT_IDDEPT_NAME

1Accounting

2Marketing

3Sales

SELECTd.DEPT_NAME,count(e.EMP_NAME)asNUM_EMPLOYEE,avg(e.EMP_SALARY)asAVG_SALARYFROMEmployee e,Department dWHEREe.DEPT_ID=d.DEPT_IDANDEMP_SALARY >5000GROUPBYd.DEPT_NAME;DEPT_NAMENUM_EMPLOYEEAVG_SALARY

Accounting18000

Marketing17000

Sales28000

From the number of employee(NUM_EMPLOYEE)column you can see that only Vijay who work forMarketingdepartment is not included in result set because his earning 5000. This example shows that, condition inWHEREclause is used to filter rows before you aggregate them and thenHAVINGclause comes in picture for final filtering, which is clear from following query, now Marketing department is excluded because it doesn't pass condition in HAVING clause i..e AVG_SALARY > 7000

SELECTd.DEPT_NAME,count(e.EMP_NAME)asNUM_EMPLOYEE,avg(e.EMP_SALARY)asAVG_SALARYFROMEmployee e,Department dWHEREe.DEPT_ID=d.DEPT_IDANDEMP_SALARY >5000GROUPBYd.DEPT_NAMEHAVINGAVG_SALARY > 7000;DEPT_NAMENUM_EMPLOYEEAVG_SALARY

Accounting18000

Sales28000

Difference between WHERE and HAVING in SQLApart from this key difference we have seen in this article, here are few more differences betweenWHEREandHAVINGclause, which is worth remembering and can be used to compare both of them :

1) Apart from SELECT queries, you can useWHEREclause withUPDATEandDELETEclause butHAVINGclause can only be used with SELECT query. For example following query, which involveWHEREclause will work but other which usesHAVINGclause will not work :

updateDEPARTMENTsetDEPT_NAME="NewSales"WHEREDEPT_ID=1;// works fine

updateDEPARTMENTsetDEPT_NAME="NewSales"HAVINGDEPT_ID=1; // errorIncorrect syntax near the keyword'HAVING'.:updateDEPARTMENTsetDEPT_NAME='NewSales'HAVINGDEPT_ID=1

2)WHEREclause is used for filtering rows and it applies on each and every row, whileHAVINGclause is used to filter groups in SQL.

3) One syntax leveldifference betweenWHEREandHAVINGclauseis that, former is used beforeGROUP BYclause, while later is used afterGROUP BYclause.

4) WhenWHEREandHAVINGclause are used together in a SELECT query with aggregate function,WHEREclause is applied first on individual rows and only rows which pass the condition is included for creating groups. Once group is created,HAVINGclause is used to filter groups based upon condition specified.

That's all on difference betweenWHEREandHAVINGclause in SQL. As I said this is very popular question and you can't afford not to prepare it. Always remember key difference betweenWHEREandHAVINGclause in SQL, ifWHEREandHAVINGclause is used together, firstWHEREclause is applied to filter rows and only after groupingHAVINGclause is applied.

Difference between Clustered Index and Non Clustered Index in SQL Server - Database Interview QuestionIn SQL Server database there are mainly two types of indexes, Clustered index and Non Clustered index, and difference between Clustered and Non Clustered index is very important from SQL performance perspective. It is also one of the most common SQL Interview question,similar to difference between truncate and delete, primary key or unique key orcorrelated vs non correlated subquery.For those, who are not aware of benefits of Index or why we use index in database, they help in making your SELECT query faster. A query with index is sometime 100 times faster than a query without index, of course depending upon how big your table is, but, you must index on columns which are frequently used inWHEREclause of SELECT query, or which forms major criterion for searching in database. For example inEmployeedatabase,EmployeeIdorEmployeeNameare common conditions to find an Employee in database. As I said, there can be eitherclustered index or non clustered index in database, former is used to decide how data is physically stored in disk and that's why there can be only one clustered index in any table. In this article, we will explore more about both of this indexes and learn some keydifference between clustered and non clustered indexfrom interview and performance perspective.

2 words on Indexes in SQLContinuing from first paragraph, Index are used to make search faster in SQL. They are mostly maintained as balanced tree (BST), where tree traversal gives you performance in order oflog(N). In case of clustered index, data is present in leaf node, so when we run a particular query, which uses clustered index, we can directly find the data by tree traversal. Query optimizer is a component of database, which decides whether to use an index or not to execute aSELECT query, or if use index then which one. You can even see, which index is used for executing your query by looking at query plan, aFULL TABLE SCANmeans no index is used and every row of table is scanned by database to find data, on the other handINDEX UNIQUE SCANorINDEX RANGE SCANsuggest use of Index for finding data. By the Index also has there own disadvantage as they makeINSERTandUPDATEquery slower and they also need space. A careful use of index is the best way to go.

Clustered vs Non Clustered Index in SQLNow we have some idea about what is Index in database and how they work, it's time to look some key differences between clustered and non clustered index in SQL Server, which is mostly true for other database as well e.g. Oracle or MySQL.

1) One of the main difference between clustered and non clustered index in SQL Server is that,one table can only have one clustered Indexbut It can have many non clustered index, approximately 250. This limitation comes from the fact clustered index is used to determines how data is stored physically in table. You should be very careful while choosing clustered index and should use columns which can be queried in range e.g.select * from Employee where EMP_ID > 20 and EMP_ID < 50. Since clustered index stores data in cluster, related data are stored together and it's easy for database to retrieve all data in one shot. This further reduces lots of disk IO which is very expensive operation. Clustered Index is also very good on finding unique values in a table e.g. queries likeselect * from Employee where EMP_ID=40; can be very fast ifEMP_IDhas clustered index on it.

2) Another key difference between Clustered Index and Non Clustered Index in database is that many relational database including SQL Server by default creates clustered index onPRIMARY KEYconstraint, if there is no clustered index exists in database and a nonclustered index is not specified while declaringPRIMARY KEYconstraint.

3) One more difference between them is that,clustered index contains data i..e rows in there leaf node, as Index is represented as BST, while nonclustered index contains pointer to data (address or rows) in there leaf node, which means one more extra step to get the data.

4) By the way there is a misconception that we can only define clustered index with one column, which is not true. You can create clustered index with multiple columns, known ascomposite index. For example inEmployeetable, a composite index onfirstnameandlastnamecan be a good clustered index, because most of the query uses this as criterion. Though you should try to minimize number of columns in clustered index for better performance in SQL Server. On related not, while declaring composite index, pay some attention to the order of columns in index, that can decide which statement will use index and which will not. In fact this is one of the mostly asked question as, does order of columns in composite index matters.

Last but not the least, pay some attention while creating clustered and non clustered index in database. Create clustered index for columns which contains unique values, are accessed sequentially, used in range queries and return large result set. Avoid creating clustered index on columns, which are update frequently because that would lead rearrangement of rows on disk level, a potentially slow operation.

That's all ondifference between clustered and nonclustered index in SQL Server database. Remember that, it's possible to create clustered index on nonPRIMARY KEYcolumn andPRIMARY KEYconstraint only creates a clustered index, if there is not already in database and a nonclustered index is not provided. Key difference is that, clustered index decides physical sorting or order of data in disk.

FRIDAY, MAY 10, 2013Difference between LEFT and RIGHT OUTER Joins in SQL - MySQL Join exampleThere are two kinds ofOUTERjoins in SQL,LEFTOUTERjoin andRIGHTOUTERjoin. Main difference betweenRIGHTOUTERjoin andLEFTOUTERjoin, as there name suggest, is inclusion of non matched rows. SineINNERjoin only include matching rows, where value of joining column is same, in final result set, butOUTERjoin extends that functionality and also include unmatched rows in final result.LEFTouter join includes unmatched rows from table written on left of join predicate. On the other handRIGHTOUTERjoin, along with all matching rows, includes unmatched rows from right side of table. In short result ofLEFTouter join isINNERJOIN+ unmatched rows fromLEFTtable andRIGHTOUTERjoin isINNERJOIN+ unmatched rows from right hand side table. Similar to difference betweenINNERjoin andOUTERjoin, difference betweenLEFTandRIGHTOUTERJOINcan be better understand by a simple example, which we will see in next section. By the way joins are very popular in SQL interviews, and along with classic questions likefinding second highest salary of employee,Inner join vs outer join or left outer join vs right outer join is commonly asked.

LEFT and RIGHT OUTER Join Example in SQLIn order to understand difference betweenLEFTandRIGHTouter join, we will use once again use classicalEmployeeandDepartmentrelationship. In this example, both of these table are connected usingdept_id, which means both have same set of data in that column, let's see data on these two table.

mysql> select * from employee;+--------+----------+---------+--------+| emp_id | emp_name | dept_id | salary |+--------+----------+---------+--------+|103 | Jack|1 |1400 ||104 | John|2 |1450 ||108 | Alan|3 |1150 ||107 | Ram|NULL |600 |+--------+----------+---------+--------+4 rows in set (0.00 sec)

mysql> select * from department;+---------+-----------+| dept_id | dept_name |+---------+-----------+|1 | Sales||2 | Finance||3 | Accounts||4 | Marketing |+---------+-----------+4 rows in set (0.00 sec)

If you look closely, there is one row in employee table which containsNULL, for which there is no entry indepartmenttable. Similarlydepartmenttable contains a department (row)Marketing,for which there is no employee in employee table. When we do aLEFTorRIGHTouter join it includes unmatched rows from left or right table. In this caseLEFTOUTERJOINshould include employee withNULLas department andRIGHTOUTER JOINshould includeMarketingdepartment. Here is example ofLEFTandRIGHTOUTERJoin in MySQL database :

mysql> select e.emp_id, e.emp_name, d.dept_name from employee e LEFT JOIN department d on e.dept_id=d.dept_id;+--------+----------+-----------+| emp_id | emp_name | dept_name |+--------+----------+-----------+|103 | Jack| Sales||104 | John| Finance||108 | Alan| Accounts||107 | Ram| NULL|+--------+----------+-----------+4 rows in set (0.01 sec)

As I said unmatched rows, i.e. row withdept_id as NULLhas included in final result anddept_namefor that row isNULL, as there is no corresponding row forNULLdept_idindepartmenttable. But note thatMarketingdepartment is not included in this result. Now, let's see example ofRIGHT OUTER JOINin MySQL, this should includeMarketingdepartment but leave out employee with NULL dept_id.

mysql> select e.emp_id, e.emp_name, d.dept_name from employee e RIGHT JOIN department d on e.dept_id=d.dept_id;+--------+----------+-----------+| emp_id | emp_name | dept_name |+--------+----------+-----------+|103 | Jack| Sales||104 | John| Finance||108 | Alan| Accounts||NULL | NULL| Marketing |+--------+----------+-----------+4 rows in set (0.00 sec)

As I said, final result set hasMarketingdepartment andemp_id,emp_nameisNULLin that row because there is no employee withdept_id=4in employee table.

Difference between LEFT and RIGHT OUTER JOIN in SQLIn short, following are some notable difference between LEFT and RIGHT outer join in SQL :

1) LEFT OUTER join includes unmatched rows from left table while RIGHT OUTER join includes unmatched rows from right side of table.

2) Result of LEFT OUTER join can be seen as INNER JOIN + unmatched rows of left able while result of RIGHT OUTER join is equal to INNER JOIN + unmatched rows from right side table.

3) In ANSI SQL, left outer join is written as LEFT JOIN while right outer join is written as RIGHT JOIN in select sql statements.

4) In Transact-SQL syntax left outer join is written as *= and right outer join is written as =*, Sybase database supports both syntax and you can write join queries in both ANSI and T-SQL syntax.

That's all ondifference between LEFT and RIGHT OUTER JOIN in SQL. We have seen example of RIGHT and LEFT join in MySQL database but since we have used ANSI syntax of OUTER joins, it's for other databases e.g. Oracle, Sybase, SQL Server and PostgreSQL as well. JOIN is a one of the most important and common concept in SQL and you should be good enough to figure out which rows will be included as a result of JOIN statement before actually running thatSELECT queryagainst any table. Some time erroneous JOIN query can bring loads of data and potentially may hang your database so beware of it.

Question 1: Oracle version 9.2.0.4.0 what does each number refers to?Answer :oracle version number refers9-Major database release number2-Database Maintenance release number0-Application server release number4-Component Specific release number0-Platform specific release number

Question 2: How do you find current date and time in oracle?Answer: This is one of the frequently asked Oracle Interview questions. I have seen this question every now and then. By the waySYSDATE functionis used in oracle to find current date and time of operating system on which the database is running return type of function is DATE

Syntax:SELECT TO_CHAR (SYSDATE, 'MM-DD-YYYY HH24:MI:SS') "Current_Date"FROM DUAL.

Question 3: Write the syntax to find current date and time in format YYYY-MM-DD.

Answer:SELECT TO_CHAR (SYSDATE, 'YYYY-MM-DD HH24:MI:SS') "Current_Date"FROM DUAL

Question 4: How will you convert a date to char in Oracle give one example

Answer :Similar to previous Oracle Interview question, this is also one of the popular question in various Oracle Interviews.to_char()function is used to convert date to character we can specify format also in which we want the output.

SELECT to_char( to_date('11-01-2012', 'DD-MM-YYYY') , 'YYYY-MM-DD') FROM dual;

or

SELECT to_char( to_date('11-01-2012, 'DD-MM-YYYY') , 'DD-MM-YYYY') FROM dual;

Question 5: What is bulk copy or BCP in oracle?Answer: BCP or bulk copy tool is one type of command line tool for unload data from database came into existence after oracle 8 .it is used to import or export data from tables and views but it will not copy structure of data same. Main advantage is fast mechanism for copying data and we can take backup of important data easily.

Question 6: What are the extensions used by oracle reportsAnswer : Oracle reports are used to make business enable to provide information of all level within or outside in secure way.REP file and RDF fileextensions are used by oracle report.

Question 7: What is Save Points in Oracle database?Answer : SAVE POINTS are used to divide a transaction into smaller parts. It enables rolling back part of a transaction. Maximum of five save points are allowed. Whenever we encounter error we can rollback from the point where we set our SAVEPOINT.This is useful for multistage transaction and conditional transaction where commit and rollback depend on certain condition. This is anothercommonly asked Oracle Interview Question and since save points are also available in other database e.g. SQL Server, some time Interviewer follow up with differences with other database as well.

Question 8: How will you convert string to a date in oracle database?Answer : This Oracle Interview questions is some time asked as follow up of previous Oracle Interview questions related to converting date to char in Oracle. By the wayto_ datefunction is used to convert string to a date function.

Syntax :to_date(string, format)Example:to_date('2012/06/12', 'yyyy/mm/dd')It will return June 12, 2012

Question 9: What is hash cluster in Oracle?Answer : This is one of my favoriteOracle Interview question. Hash cluster is one of the techniques to store the table in a hash cluster to improve the performance of data retrieval .we apply hash function on the table rows cluster key value and store in the hash cluster. All rows with the same hash key value are stores together on disk.key value is same like key of index cluster ,for data retrieval Oracle applies the hash function to the row's cluster key value.

Question 10: What is Snap shot in Oracle database?Answer : Snapshots are read-only copies of a master table located on a remote node which is periodically refreshed to reflect changes made to the master table.

Thats all on this list ofOracle Interview questions and answers. This can be a good recap before appearing to any programming job interview, where questions from Oracle database is expected. You may want to prepare some SQL Interview question as well along with these Oracle questions, as questions related to SQL query e.g.How to find second highest salary in SQLorHow to find duplicate records in tableis quite popular on various Oracle Interviews.

THURSDAY, NOVEMBER 22, 2012How to join three tables in SQL query MySQL ExampleThree table JOIN Example SQLJoining three tables in single SQL query can be very tricky if you are not good with concept of SQL Join. SQL Joins have always been tricky not only for new programmers but for many others,who are inprogrammingand SQL for more than 2 to 3 years. There are enough to confuse someone on SQL JOIN ranging from various types of SQL JOIN like INNER and OUTER join, LEFT and RIGHT outer join, CROSS join etc. Between all of these fundamentals, What is most important about Join is, combining multiple tables. If you need data from multiple tables in oneSELECT queryyou need to use eithersubqueryor JOIN . Most of times we only join two tables likeEmployeeandDepartmentbut some time you may require to join more than two tables and a popular case is joining three tables in SQL. In case of joining three tables table 1 relates to table 2 and than table 2 relates to table 3. If you look at closely you find that table 2 is a joining table which containsprimary keyfrom both table 1 and table 2. As I said it can be extremely confusing to understand join of three or more tables. I have found that understanding table relationship asprimary keyand foreign key helps to alleviate confusion than the classical matching row paradigm. SQL Join is also a very popular topic in SQL interviews and there is always been some questions from Join like Difference between INNER and OUTER JOIN,SQL query with JOIN e.g.EmployeeDepartmentrelationship andDifference between LEFT and RIGHT OUTER JOIN etc. In short this is one of the most important topic in SQL both from experience and interview point of view.

Three table JOIN syntax in SQLHere is a general SQL query syntax to join three or more table. This SQL query should work in all major relation database e.g. MySQL, Oracle, Microsoft SQLServer, Sybase and PostgreSQL :

SELECTt1.col,t3.colFROMtable1 join table2ONtable1.primarykey=table2.foreignkey join table3ONtable2.primarykey=table3.foreignkey

We first join table 1 and table 2 which produce atemporary tablewith combined data from table1 and table2,which is then joined to table3. This formula can be extended for more than 3 tables to N tables, You just need to make sure that SQL query should haveN-1join statement in order to join N tables. like for joining two tables we require 1 join statement and for joining 3 tables we need 2 join statement.

SQL Query to JOIN three tables in MySQLIn order to better understandjoining of 3 tables in SQL querylet's see an example.Consider popular example ofEmployeeandDepartmentschema. I our case we have used alink tablecalledRegisterwhich link or relate bothEmployeetoDepartment. Primary key ofEmployeetable(emp_id)is foriegn key inRegisterand similarly primary key ofDepartmenttable(dept_id)is foreign key inRegistertable.

In order towrite an SQL query to print employee name and department namealong side we need tojoin 3 tables. First JOIN statement will joinEmployeeandRegisterand create a temporary table which will havedept_idas another column. Now second JOIN statement will join this temp table withDepartmenttable ondept_idto get desired result. Here is the completeSELECT SQL query exampleto join 3 tables and it can be extended to join more than 3 or N tables.

mysql>SELECT*FROMEmployee;+--------+----------+--------+|emp_id|emp_name|salary|+--------+----------+--------+|1 |James |2000||2 |Jack |4000||3 |Henry |6000||4 |Tom |8000|+--------+----------+--------+4rowsINSET(0.00sec)

mysql>SELECT*FROMDepartment;+---------+-----------+|dept_id|dept_name|+---------+-----------+|101 |Sales ||102 |Marketing||103 |Finance |+---------+-----------+3rowsINSET(0.00sec)

mysql>SELECT*FROMRegister;+--------+---------+|emp_id|dept_id|+--------+---------+| 1| 101|| 2| 102|| 3| 103|| 4| 102|+--------+---------+4rowsINSET(0.00sec)

mysql>SELECTemp_name,dept_nameFROMEmployee eJOINRegister rONe.emp_id=r.emp_idJOINDepartment dONr.dept_id=d.dept_id;+----------+-----------+|emp_name|dept_name|+----------+-----------+|James |Sales ||Jack |Marketing||Henry |Finance ||Tom |Marketing|+----------+-----------+4rowsINSET(0.01sec)

If you want to understand it even more better than try joining tables step by step. So instead of joining 3 tables in one go, first join 2 tables and see how the result table will look like. Thats all on How to join three tables in one SQL query in relational database. By the way in this SQL JOIN Example we have used ANSI SQL and it will work in other relational database as well e.g. Oracle , SQL Server, Sybase, PostgreSQL etc. Let us know if you face any issue while running this 3 table JOIN query in any other database.

10 Example Queries of SQL Select CommandSelect command in SQLis one of the most powerful and heavily used commands. This is I guess the first command any one learn in SQL even before CREATE which is used to create table in SQL.SELECT is used in SQLto fetch records from database tables and you can do a lot many things using Select. For example you can select all records, you can select few records based on condition specified in WHERE clause, select all columns using wild card (*) or only selecting few columns by explicitly declaring them in query.

In thisSELECT SQL command tutorial we will see someexamples of select commandor Select Statement and will write sql queries to demonstrate the result. We will use following table and data for our SQL query examples, one table represent Stocks listed in various market and other table contains Details of market e.g. Country. MySQL is my favorite RDBMS and great for learning purpose you can download MySQL and start working on it. My suggestion is to use command line interface for writing queries instead of using GUI e.g. SQL Developer orMySQLquery tool. Command line is best for learning and real fun of writing SQL query is only on command prompt.

mysql> select * from STOCK;+---------+-------------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+---------+-------------------------+--------------------+| 6758.T | Sony | T || GOOG.O | Google Inc | O || GS.N | Goldman Sachs Group Inc | N || INFY.BO | InfoSys | BO || VOD.L | Vodafone Group PLC | L |+---------+-------------------------+--------------------+5 rows in set (0.00 sec)

mysql> select * from MARKET;+------+-------------------------+---------------+| RIC | NAME | COUNTRY |+------+-------------------------+---------------+| T | Tokyo Stock Exchange | Japan || O | NASDAQ | United States || N | New York Stock Exchange | United States || BO | Bombay Stock Exchange | India |+------+-------------------------+---------------+4 rows in set (0.00 sec)

SQL SELECT command query exampleshere are some of my favorite select clause examples which explores different ways one can use select command for reporting purpose and display results.1) Finding how many rows in tables

mysql> select count(*) from STOCK;+----------+| count(*) |+----------+| 5 |+----------+

2) Finding all records from tables; we are using wildcard start * for getting all columns.

mysql> select * from STOCK;+---------+-------------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+---------+-------------------------+--------------------+| 6758.T | Sony | T || GOOG.O | Google Inc | O || GS.N | Goldman Sachs Group Inc | N || INFY.BO | InfoSys | BO || VOD.L | Vodafone Group PLC | L |+---------+-------------------------+--------------------+5 rows in set (0.00 sec)

3. Selecting few records based on some condition from tables in SQL

mysql> select * from STOCK where RIC='GOOG.O';+--------+------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+--------+------------+--------------------+| GOOG.O | Google Inc | O |+--------+------------+--------------------+

4. How to select few columns instead of all columns?Instead of using start wild-card just give name of interested columns to SELECT clause.

mysql> select COMPANY from STOCK where RIC='GOOG.O';+------------+| COMPANY |+------------+| Google Inc |+------------+

5. Select distinct (unique) records from ColumnsDistinct keyword is used to show only unique records it will not show any duplicate values.

mysql> select distinct LISTED_ON_EXCHANGE from Stock;+--------------------+| LISTED_ON_EXCHANGE |+--------------------+| T || O || N || BO || L |+--------------------+

6. Selecting value with condition based on less than, greater than (>, =, select * from Stock where RIC > 'I';+---------+--------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+---------+--------------------+--------------------+| INFY.BO | InfoSys | BO || VOD.L | Vodafone Group PLC | L |+---------+--------------------+--------------------+

7. Combining condition using logical operator AND & ORAND and OR Can be effectively used to combine two conditions on WHERE clause and gives you lot of flexibility to write SQL query.

mysql> select * from Stock where RIC 'G';+--------+-------------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+--------+-------------------------+--------------------+| GOOG.O | Google Inc | O || GS.N | Goldman Sachs Group Inc | N |+--------+-------------------------+--------------------+

You can put any number of AND, OR conditions on WHERE Clause, some time things become quite easy when you combine AND, OR in SQL.

8. How to find records which is not null using keyword NULL and IS NULLNULL is very tricky in SQL; NULL means anything which doesn't have value. NULL is not "null" which will be treated as text.To demonstrate this we will insert a Stock which is not listed on any Market yet.

mysql> select * from STOCK;+---------+-------------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+---------+-------------------------+--------------------+| 6758.T | Sony | T || GOOG.O | Google Inc | O || GS.N | Goldman Sachs Group Inc | N || INDIGO | INDIGO Airlines | NULL || INFY.BO | InfoSys | BO || VOD.L | Vodafone Group PLC | L |+---------+-------------------------+--------------------+6 rows in set (0.00 sec)

You See there is only one row who has LISTED_ON_EXCHANGE null, we will now see count using NULL and IS NULL which will verify this result.

mysql> select count(*) from STOCK where LISTED_ON_EXCHANGE IS NULL;+----------+| count(*) |+----------+| 1 |+----------+1 row in set (0.00 sec)

mysql> select count(*) from STOCK where LISTED_ON_EXCHANGE IS NOT NULL;+----------+| count(*) |+----------+| 5 |+----------+1 row in set (0.00 sec)

mysql> select count(*) from STOCK;+----------+| count(*) |+----------+| 6 |+----------+1 row in set (0.00 sec)

9. SELECT Statement using BETWEEN and NOT BETWEEN

As name suggest BETWEEN is used to get data between a ranges.

mysql> select * from Stock where RIC BETWEEN 'G' AND 'I';+--------+-------------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+--------+-------------------------+--------------------+| GOOG.O | Google Inc | O || GS.N | Goldman Sachs Group Inc | N |+--------+-------------------------+--------------------+

10. Pattern matching in SQL queries using LIKE and NOT LIKELIKE is a pattern matching operator and used to find records which are not exact match but probable match.

mysql> select * from Stock where RIC LIKE 'V%';+-------+--------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+-------+--------------------+--------------------+| VOD.L | Vodafone Group PLC | L |+-------+--------------------+--------------------+

NOT LIKE is opposit of LIKE and display records which are not probable match.mysql> select * from Stock where RIC NOT LIKE 'V%';+---------+-------------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+---------+-------------------------+--------------------+| 6758.T | Sony | T || GOOG.O | Google Inc | O || GS.N | Goldman Sachs Group Inc | N || INDIGO | INDIGO Airlines | NULL || INFY.BO | InfoSys | BO |+---------+-------------------------+--------------------+

11. IN and NOT ININ is another useful SQL operator we can use alongside SELECT. it provides set of values which can be used in WHERE cluase.

mysql> select * from Stock where RIC in ('GS.N' , 'INFY.BO');+---------+-------------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+---------+-------------------------+--------------------+| GS.N | Goldman Sachs Group Inc | N || INFY.BO | InfoSys | BO |+---------+-------------------------+--------------------+

12. Sorting ResultSet in SQL using ORDER BY, ASC, DESCOrder by is used to sort records in result set returned by SELECT clause. By default it list in Ascending order but we can use either ascending or descending using specifier ASC and DESC.

mysql> select * from Stock order by COMPANY;+---------+-------------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+---------+-------------------------+--------------------+| GS.N | Goldman Sachs Group Inc | N || GOOG.O | Google Inc | O || INDIGO | INDIGO Airlines | NULL || INFY.BO | InfoSys | BO || 6758.T | Sony | T || VOD.L | Vodafone Group PLC | L |+---------+-------------------------+--------------------+

14. Selecting data from multiple tables by using JOIN in SQLJoin in SQL is powerful concept which allows you to select data from multiple tables. You can generate report where data is accumulated from different tables based on conditions specified in Join statement.

Suppose you need to display list of Records and Name of Market where they are listed. Here name of Stock in STOCK table while name of exchange in MARKET table. We need to join both of them to display this report.

mysql> select s.RIC, m.NAME from Stock s, Market m where s.LISTED_ON_EXCHANGE=m.RIC;+---------+-------------------------+| RIC | NAME |+---------+-------------------------+| 6758.T | Tokyo Stock Exchange || GOOG.O | NASDAQ || GS.N | New York Stock Exchange || INFY.BO | Bombay Stock Exchange |+---------+-------------------------+

Above method is called implicit Join an d This query can also be written by using explicit join style which uses ON clause to join tables.

mysql> select s.RIC, m.NAME from Stock s INNER JOIN Market ON m I s.LISTED_ON_EXCHANGE=m.RIC;

15. Calling function on SELECT clause e.g. displaying current date

mysql> select now();+---------------------+| now() |+---------------------+| 2011-10-13 10:25:47 |+---------------------+

16. Doing calculation using SELECT CLAUSEYou can perform some basic calculation using SELECT clause e.g addition, subtraction, multiplication, division etc.

mysql> select 1+2;+-----+| 1+2 |+-----+| 3 |+-----+

17. SELECT data from one row till another row like PagingIf you are thinking to implement paging and getting data from specified row you can do this easily in Mysql by using LIMIT clause.

mysql> select * from Stock order by COMPANY LIMIT 0,2;+--------+-------------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+--------+-------------------------+--------------------+| GS.N | Goldman Sachs Group Inc | N || GOOG.O | Google Inc | O |+--------+-------------------------+--------------------+

Here first parameter '0' says start from first record and '2' says get 2 record only.

18. Selecting data from result of another query by using derived table.Sometime information needed to produce final SELCT result comes from another query and act as table for outer SELECT statement. This table also called Derived table

mysql> select RIC from (select s.RIC, m.NAME from Stock s, Market m where s.LISTED_ON_EXCHANGE=m.RIC) t where RIC > 'G'

+---------+| RIC |+---------+| GOOG.O || GS.N || INFY.BO |+---------+

Some Important point about SELECT command in SQL:

So Far we have seen different examples of SELECT clause in SQL which will enable you to take full advantage of SELECT while writing SQL queries. Here I have listed some important points which you should consider while writing SQL query not just SELECT but with any other keyword also.

1) Most often we use SELECT Operator with WHERE Clause, try to use column which has index on WHERE clause.Using a non index column on WHERE clause can slow your query drasticallyand effect would be more visible when your table data increases. a example with 1 Million records query without index was taking 80 second while after index it just took .3 second, whopping 260% increase in speed.

2) If you don't need all columns,dont use * wild card. SELECT query with few columns are slightly faster than all columns.

3) If you are retrieving data from large table,do a count (*) check before firing actual select query, this will give you en estimate of how many records you are about to get and how much time it could take.

4) You canintroduce new columns in result set of SELECT Query by using keyword "AS"as shown in below example. Very useful for displaying calculated value e.g. average or percentage.

5) Alwaysuse IS NULL or NULLfor including or excluding values which could be null.Dont use 'null'that will be treated as text.

6) While writing SQL query not just SELECT, its good practice towrite keyword in small caseandTABLES and COLUMNS in capital. So that they will stand out from whole query and makes query more readable.

That's all onSQL Select command examples, I tried to cover good number of select command example to provide an overview what SELECT statement can do. If you know any good select example in sql please share.

WEDNESDAY, OCTOBER 19, 2011Difference between Truncate and Delete command in SQL - Interview Questions with ExampleDelete and truncate command in SQLTruncate and delete in SQLare two commands which is used to remove or delete data from table. Though quite basic in nature both sql commands can create lot of trouble until you are familiar with details before using it. Difference betweenTruncate and deleteare not just important to understand perspective but also a very popularSQL interview topicwhich in my opinion a definite worthy topic. What makes them tricky is amount of data. Since most of Electronic trading system stores large amount of transactional data and some even maintain historical data, good understanding ofdelete and truncate commandis required to effectively work on those environment.I have still seen people firing delete command just to empty a table with millions of records which eventually lock the whole table for doing anything and take ages to complete or Simply blew log segment or hang the machine.

Most ofenterprise stock trading systemmaintains two kind of database one transactional and other static. Transactional data is for day by day records which need to be purge at end of data or moved to historical data so that application can make a fresh start another day. If you need to work on such large set of data, my advice is to get clear and complete knowledge of delete and truncate command, along with there differences and when to use which command to remove data or purge tables.

In this article we will seewhere to use truncate in SQLand where to use delete in SQL, How to use truncate or delete and what danger or harm they can create if not used carefully along withdifference between truncate and delete in SQL.

What is Truncate command in SQLUse truncate table if you need to delete all rows, since truncate doesn't allow you to specify WHERE clause. truncate removes data by deallocating space used by table which removes lot of overhead in terms of logging and locking and that's why truncate is faster than delete.What you need to take care is rollback, data deleted by truncate can not be rolled back until data server specifically supports it e.g. MSSQL Server which allows tocommit or rollback truncate tablestatement transactional. Another caveat with truncate table statement is that it doesn't fire a trigger and you can not truncate a table when a foreign key references any column to the table to be truncated. Only situation I see which is perfect for using truncate is purging tables with huge data, though there is another solution exists to drop table and recreated it if that make sense.

Example of truncate command in SQL

truncate table Orders;//Order table shouldn't have a column which is foreign key on other table

What is Delete command in SQLDelete is another sql command available for removing records from table. Delete is even more flexible than truncate like it provides support to WHERE Clause which can be use to remove selective data. It logs each row which allows operation to be rolled back and it also fires triggers. One disadvantage of using delete is speed and locking. Delete acquires lock on table and its also very slow operation because of logging, which makes it unsuitable for removing records from large tables. One workaround for this is batch-delete in which you remove batch of records instead on one record at a time. Delete is most suitable fore removing selective data and use it where you want torollback transaction in database. Its not useful to purge large amount of data from tables and should not be used, otherwise it could lock the table for very long time, blew log segment and can take ages to complete.

Example of delete command in SQL

delete * from Orders;//delete all row from Orders, should not be used if Orders is largedelete * from Orders where Symbol="MSFT.NQ"//delete all orders where symbol is MSFT.NQ

Difference between truncate and delete command in SQLThis is an important point to understand before using truncate or delete on production environment, or writing any script which purges data from tables.

1.truncate is fast delete is slow.2. truncate doesn't do logging delete logs on per row basis.3. rollback is possible with delete not with truncate until specifically supported by vendor.4. truncate doesn't fire trigger, delete does.5. Don't delete, truncate it when it comes to purge tables.6. truncate reset identity column in table if any, delete doesn't.7. truncate is DDL while delete is DML (use this when you are writing exam)8. truncate doesn't support where clause, delete does.

So finally if you have table with huge data and want to empty itdont Delete, truncate it

Interview questions on truncate and delete in SQLTruncate and delete both are popular interview topics and there is always some question on these commands in SQL interview. Here I am listing some ofSQL interview questionsbased on delete and truncate command in SQL, you can find answer in this article itself or by google.

1) If you have table which contains large amount of data which command will you use for removing data, truncate or delete?

2) What are differences between truncate and delete?

3) Which one is fast truncate or delete?

4) What is disadvantage of using truncate in sql?

5) How will you delete data if truncate is not supported and log segment is also not big enough to support complete delete?

6) Is there any way to remove data other than truncate and delete in SQL?

How to find second highest or maximum salary of Employee in SQL - Interview questionHow to find second highest or second maximum salary of an Employee is one of the most frequently asked SQL interview question similar tofinding duplicate records in tableandwhen to use truncate vs delete. There are many ways to find second highest salary based upon which database you are using as different database provides different feature which can be used to findsecond maximumorNth maximumsalary of employee. Well this question can also be generalized with other scenario like finding second maximum age etc. In this SQL tutorial we will see different exampleofSELECT SQL querytofind second highest salary independent of databases or you may call in ANSI SQL and other SQL queries which uses database specific feature to find second maximum salary.

SQL query to find second maximum salary of EmployeeIn this section we will write SQL query to get second highest salary of Employee. Before writing query its good to be familiar with schema as well as data in table. Here is the Employee table we will be using this SQL example:

mysql>SELECT*FROMEmployee;+--------+----------+---------+--------+|emp_id|emp_name|dept_id|salary|+--------+----------+---------+--------+|1 |James |10 |2000||2 |Jack |10 |4000||3 |Henry |11 |6000||4 |Tom |11 |8000|+--------+----------+---------+--------+4rowsINSET(0.00sec)

If you look data, you will find that second maximum salary in this case is 6000 and employee name is Henry. Now lets see some SQL example to find out this second maximum salary.

Second maximum salary using sub query and IN clauseSub queries in SQL are great tool for this kind of scenario, here we first select maximum salary and then another maximum excluding result of subquery. To learn more about Subqueryseecorrelate and non-correlate subquery in SQL

mysql>SELECTmax(salary)FROMEmployeeWHEREsalaryNOTIN(SELECTmax(salary)FROMEmployee);+-------------+|max(salary)|+-------------+| 6000|+-------------+1rowINSET(0.00sec)

Here is another SQL query to find second highest salary using subquery and < operator instead of IN clause:

mysql>SELECTmax(salary)FROMEmployeeWHEREsalarySELECTsalary FROM(SELECTsalaryFROMEmployeeORDERBYsalaryDESCLIMIT2)ASempORDERBYsalaryLIMIT1;+--------+|salary|+--------+|6000|+--------+1rowINSET(0.00sec)

Thats on How to find second highest salary of Employee using SQL query. This is good question which really test your SQL knowledge, its not tough but definitely tricky for beginners. As follow up question you can ask him to find third maximum salary or Nth maximum salary as well.

FRIDAY, NOVEMBER 25, 2011Database Transaction Tutorial in SQL with Example for BeginnersDatabase transactionis an important concept to understand while working in database and SQL. Transaction in database is required to protect data and keep it consistent when multiple users access the database at same time. In thisdatabase transaction tutorialwe will learn what is transaction in database, why do you needtransaction in database, ACID properties of database transaction and an example of database transaction along with commit and rollback. Almost all vendors like Oracle, MySQL, SQL Server or Sybase provide transaction facility though MySQL only provide it for certain storage engine like InnoDB and BDB and not for MyISAM.What is transaction in database?Database transaction is collection of SQL queries which forms a logical one task. For transaction to be completed successfully all SQL queries has to run successfully. Database transaction executes either all or none, so for example if your database transaction contains 4 SQL queries and one of them fails then change made by other 3 queries will be rolled back. This way your database always remain consistent whether transaction succeeded or failed. Transaction is implemented in database usingSQL keyword transaction, commit and rollback. Commit writes the changes made by transaction into database and rollback removes temporary changes logged in transaction log by database transaction.

Database Transaction tutorialWhy transaction is required in databaseDatabase is used to store data required by real life application e.g. Banking, Healthcare, Finance etc. All your money stored in banks is stored in database, all your shares of DMAT account is stored in database and many application constantly work on these data. In order to protect data and keep it consistent any changes in this data needs to be done in transaction so that even in case of failure data remain in previous state before start of transaction. Consider a Classical example of ATM (Automated Tailor Machine); we all use to withdraw and transfer money by using ATM. If you break withdrawal operation into individual steps you will find:

1) Verify account details.2) Accept withdrawal request3) Check balance4) Update balance4) Dispense money

Suppose your account balance is 1000$ and you make a withdrawal request of 900$. At fourth step your balance is updated to 900$ and ATM machine stops working due to power outageOnce power comes back and you again tried to withdraw money you surprised by seeing your balance just 100$ instead of 1000$. This is not acceptable by any person in the world :) so we need transaction to perform such task. If SQL statements would have been executed inside transaction in database balance would be either 100$ until money has been dispensed or 1000$ if money has not been dispensed.

ACID Properties of database transactionThere are four importantproperties of database transactionsthese are represented by acronym ACID and also calledACID properties or database transactionwhere:

A stands forAtomicity, Atom is considered to be smallest particle which can not be broken into further pieces.database transaction has to be atomic means either all steps of transaction completes or none of them.

C stands forConsistency, transaction must leave database in consistent state even if it succeed or rollback.

I is forIsolationTwo database transactions happening at same time should not affect each other and has consistent view of database. This is achieved by using isolation levels in database.

D stands forDurabilityData has to be persisted successfully in database once transaction completed successfully and it has to be saved from power outage or other threats. This is achieved by saving data related to transaction in more than one places along with database.

When to use database transactionWhenever any operation falls under ACID criteria you should use transactions. Many real world scenarios require transaction mostly in banking, finance and trading domain.

How to implement transaction in SQLDatabase transaction is implemented in SQLusing three keywords start transaction, commit and rollback.once you type start transaction, database starts a transaction and execute all subsequent SQL statements in transaction and keep track of all of them to either commit or rollback changes.Commitkeywords saves then changes made by transaction into database and after commit change is normally visible to other transaction though is subject toisolation level. In case you encountered any error while executing individual sql statements inside database transaction, you can rollback all your changes by executing "rollback" command.

Database Transaction ExampleTo understand database transaction better let's see a real life example of transaction in database. For this example we will assume we have an Account table which represent a Bank Account and we will transfer money from one account to another account

Request: transfer 900$ from Account 9001 to 9002

start transactionselect balance from Account where Account_Number='9001';select balance from Account where Account_Number='9002';update Account set balance=balance-900 here Account_Number='9001' ;update Account set balance=balance+900 here Account_Number='9002' ;commit;//if all sql queries succedrollback;//if any of Sql queries failed or error

Database transaction in MySQLIn my previousmysql command tutorialsI have talked aobut different databse storage engines available in mysql e.g. myISAM or InnoDB. Not all mysql engines supports transaction in order to make transaction works in mysql you either need to use InnoDB or BDB Engine. You can specify engige while creating table in mysql or you can also change your engine in mysql by using ALTER keyword. For example"ALTER TABLE tablename TYPE=InnoDB;

Important point about database transaction1.Database transactionis nothing but a set of SQL statement.

2. Transaction in database is either all or none means either all SQL statement success or none.

3. Its good practice to execute sql query inside transaction and commit or rollback based on result but you need to be little careful with transaction log. To faciliate rollback and commit every sql query which executed inside database transaction is written into transaction log and size of transaction log can grow significantly if don't commit or rollback for longtime.

4. Effect of two simulteneous database transaction into data is controlled by using Isolation level. Isolation level is used to separate one database transaction with other and currently there are four databse isolation levels:1) Read UncommitedThis is lowest level of databse isolation level in this one database transaction can see changes made by other databse transaction which is not yet commited. This can allow you dirty read so quite dangerous.2)Read CommitedThis is sligltly better where one database transaction only sees commited changes by other database transaction. But this is also not safe and can lead you to non-repeatable reads problem.3)Repeatable Reads4)SerializableHighest level of database isolation level. In this alldatabase transactionsare totally isolated with other database transaction.though this is safe but this safety can cause significant performance hit.

5. MyISAM storage engine in MySQL doesn't support transaction. In order to make transaction works in MySQL use InnoDB.

6. Databse transaction should follow ACID properties.

Thats all for now on database transaction tutorial, I will add more useful points about transaction in databse as I come across or recall, you can also provide your input and issues face during transaction in database on different RDBMS e.g. Oracle, MySQL, MSSQL Server or Sybase etc.

FRIDAY, DECEMBER 14, 2012How to find duplicate records in a table on database - SQL tipsHow to find duplicate records in table is a popular SQL interview question which has been asked as many times asdifference between truncate and deletein SQL or finding second highest salary of employee. Both of these SQL queries are must know for any one who is appearing on anyprogramming interviewwhere some questions on database and SQL are expected. In order tofind duplicate records in database tableyou need to confirm definition of duplicates, for example in belowcontacttable which is suppose to storenameandphone numberof contact, a record is considered to be duplicate if both name and phone number is same but unique if either of them varies. Problem of duplicates in database arise when you don't have aprimary key or unique keyon database and that's why its recommended to have a key column in table. Anyway its easy to find duplicate records in table by usinggroup by clauseof ANSI SQL. Group by clause is used to group data based upon any column or a number of columns. Here in order to locate duplicate records we need to usegroup by clauseon bothnameandphoneas shown in secondSQL SELECT query example. You can see in first query that it listed Ruby as duplicate record even though both Ruby have different phone number because we only performed group by on name. Once you have grouped data you can filter out duplicates by usinghaving clause. Having clause is counter part ofwhere clausefor aggregation queries. Just remember to provide temporary name tocount()data in order to use them in having clause.

SQL Query to find duplicate records in a table in MySQLIn this section we will see SQL query which can be used to locate duplicate records in table. As explained in previous section, definition of duplicate depends upon business rules which must be used in group by clause. In following query we have usedSELECT queryto select all records fromContactstable. Here James, Johnny, Harry and Ron are duplicated four times.

mysql> select * from Contacts;+-------+----------+| name| phone|+-------+----------+|James | 80983243||Johnny | 67543212 || Harry | 12341234 || Ron| 44446666||James | 80983243 || Johnny | 67543212 || Harry | 12341234 || Ron| 44446666|| James | 80983243 || Johnny | 67543212 || Harry | 12341234 || Ron| 44446666 || James | 80983243 || Johnny | 67543212 || Harry | 12341234 || Ron| 44446666 || Ruby|8965342 || Ruby|6888342 |+-------+----------+18 rows in set (0.00 sec)

Following SELECT query willonly find duplicates records based on namewhich might not be correct if two contact of same but different numbers are stored, as in following result set Ruby is shown as duplicate which is incorrect.

mysql> select name, count(name) from contacts group by name;+-------+-------------+| name| count(name) |+-------+-------------+| Harry |4 || James |4 || Johnny |4 || Ron|4 || Ruby|2 |+-------+-------------+5 rows in set (0.00 sec)

This is the correct way of finding duplicate contacts at it lookboth name and phone numberand only print duplicate if both name and phone is same.

mysql> select name, count(name) from contacts group by name, phone;+-------+-------------+| name| count(name) |+-------+-------------+| Harry |4 || James |4 || Johnny |4 || Ron|4 || Ruby|1 || Ruby|1 |+-------+-------------+6 rows in set (0.00 sec)

having clause in SQL querywill filter duplicate records from non duplicate records. As in following query it print all duplicate records and how many times they are duplicated in table.

mysql> select name, count(name) as times from contacts group by name, phone having times>1;+-------+-------+| name| times |+-------+-------+| Harry |4 || James |4 || Johnny |4 || Ron|4 |+-------+-------+4 rows in set (0.00 sec)

That's all on how to find duplicate records in table, These SQL queries will work on all database likeMySQL,Oracle, SQL Server and Sybase as it only uses ANSI SQL and doesn't use any database specific feature. Another interesting SQL query interview question is "How to delete duplicate records from table" which we will see in another post.What is difference between primary key and unique key in table - SQL databaseprimary key vs unique key in SQLprimary key and unique key are two important concept in relational database, and used to uniquely identify a row in a table. Both primary key and unique key can identify a row uniquely but there are some subtle difference between them which we will see in this article. In fact primary key vs unique is a popular SQL interview questions along with classics liketruncate vs deleteandHow to manage transaction in database, mostly asked to fresher and 2 to 3 years experience guys in any programming language. SQL is not just limited to any DBA or PLSQL developer but its an important skill even for Java programmer and you can expect SQL interview question even in manyJava interviews. Some time programmer also confuse between foreign key and unique key, which is primary key of other table in relation.

Difference between primary key and unique key in SQLAs I said both primary and unique key uniquely identifies each row in table but there are some subtle difference between them. here are some of them :

1)Unique keyin a tablecan benull, at-least one butprimary keycan not be nullin any table in relation database like MySQL , Oracle etc.

2) Primary key can be combination of more than one unique keys in same table.

3) There can be only oneprimary keyper table in relation database e.g.MySQL, Oracle or Sybase but there can be more than oneunique keyper table.

4) Unique key is represented using unique constraint while primary key is created using primary key constraint in any table and it's automatically gets unique constraint.

5) Many database engine automatically puts clustered index on primary key and since you can only have one clustered index per table, its not available to any other unique key at same time.

These were some of thedifference between primary key and unique key in SQLor any table. Its's one of those SQL interview questions which you don't like to miss before going for anyprogramming interviewor any database, SQL interview.

SATURDAY, JULY 7, 2012SubQuery Example in SQL Correlated vs NoncorrelatedSubQuery in SQLis a query inside another query. Some time to get a particular information from database you may need to fire two separate sql queries, subQuery is a way to combine or join them in single query. SQL query which is on inner part of main query is called inner query while outer part of main query is called outer query. for example in below sql query

SELECTnameFROMCityWHEREpincodeIN(SELECTpincodeFROMpinWHEREzone='west')

section not highlighted isOUTER querywhile section highlighted with grey isINNER query. In this SQL tutorial we will see both Correlated and non correlated sub-query and there examples, somedifferences between correlated and noncorrelated subqueriesand finallysubquery vs joinwhich is classic debatable topic in SQL. By the way this SQL tutorial is next in series of SQL and database articles in Javarevisited liketruncate vs deleteand10 examples of SELECT queries. If you are new here then you may find those examples interesting.

SubQuery Rules in SQLLike any other concept in SQL, subquery also has some rules and you can only embed one query inside another by following rules :1. subquery can be used in insert statement.2. subquery can be used in select statement as column.3. subquery should always return either a scaler value if used with where clause or value from a column if used withINorNOT INclause.

Before going to understandnon-correlated and correlated subquery, lets see the table and data which we are going to use in this example. Until you have an understanding of how table look like and what kind of data it stores its little difficult to understand queries. In this subquery example we will use two tableStockandMarket.Stockholds different stocks andMarketholds all stock exchanges in the world.

mysql> select * from stock;+---------+-------------------------+--------------------+| RIC | COMPANY | LISTED_ON_EXCHANGE |+---------+-------------------------+--------------------+| 6758.T | Sony | T || GOOG.O | Google Inc | O || GS.N | Goldman Sachs Group Inc | N || INDIGO | INDIGO Airlines | NULL || INFY.BO | InfoSys | BO || VOD.L | Vodafone Group PLC | L |+---------+-------------------------+--------------------+6 rows in set (0.00 sec)

mysql> select from Market;+------+-------------------------+---------------+| RIC | NAME | COUNTRY |+------+-------------------------+---------------+| T | Tokyo Stock Exchange | Japan || O | NASDAQ | United States || N | New York Stock Exchange | United States || BO | Bombay Stock Exchange | India |+------+-------------------------+---------------+4 rows in set (0.00 sec)

Noncorrelated subquery in SQLThere are two kind of subquery in SQL one is called non-correlated and other is called correlated subquery. In non correlated subquery,inner query doesn't depend on outer queryand can run as stand alone query.Subquery used along-with IN or NOT IN sql clause is good examples of Noncorrelated subquery in SQL. Let's anoncorrelated subquery exampleto understand it better.

NonCorrelated Subquery Example:Lets see the query Find all stocks from Japan, If we analyze this query we know that stock names are stored inStocktable whileCountry nameis stored inMarkettable, so we need to fire two query first to getRICfor Japanese market and than all stocks which is listed on thatMarket. we can combine these two queries into one sql query by using subquery as shown in below example:

mysql>SELECTCOMPANYFROMStockWHERELISTED_ON_EXCHANGE=(SELECTRICFROMMarketWHERECOUNTRY='Japan');+---------+|COMPANY|+---------+|Sony |+---------+1rowINSET(0.02sec)

Here part which is inside bracket is called inner query or subquery. As you see in this example of subquery,inner query can run aloneand its not depended on outer query and that's why its calledNonCorrelated query.

NonCorrelated Subquery Example with IN Clause SQLNonCorrelated subquery are used along-with IN and NOT IN clause. here is an example of subquery with IN clause in SQL.SQL query: Find all stocks from United States and India

mysql>SELECTCOMPANYFROMStockWHERELISTED_ON_EXCHANGEIN(SELECTRICFROMMarketWHERECOUNTRY='United States'ORCOUNTRY='INDIA');+-------------------------+|COMPANY |+-------------------------+|Google Inc ||Goldman SachsGROUPInc||InfoSys |+-------------------------+

When Subquery is used along-withIN or NOT IN Clauseit returns result from one column instead of Scaler value.

Correlated SubQuery in SQLCorrelated subqueriesare the one in whichinner query or subquery reference outer query. Outer query needs to be executed before inner query. One of the most commonexample of correlated subqueryis using keywordsexitsandnot exits. An important point to note is thatcorrelated subqueries are slower queriesand one should avoid it as much as possible.

Example of Correlated Subquery in SQLHere is an example of Correlated subquery Return all markets which has at least one stock listed on it.

mysql>SELECTm.NAMEFROMMarket mWHEREm.RIC=(SELECTs.LISTED_ON_EXCHANGEFROMStock sWHEREs.LISTED_ON_EXCHANGE=m.RIC);

+-------------------------+|NAME |+-------------------------+|Tokyo Stock Exchange ||NASDAQ ||New York Stock Exchange||Bombay Stock Exchange |+-------------------------+4rowsINSET(0.00sec)

Here inner query will execute for every Market as RIC will be changed for every market.

Difference between Correlated and NonCorrelated SubqueryNow we have seen correlated and noncorrelated subqueries and there example its much easier to understanddifference between correlated vs noncorrelated queries. By the way this is also one of the popular sql interview question and its good to know few differences:

1.In case ofcorrelated subqueryinner query depends on outer query while in case of noncorrelated query inner query or subquery doesn't depends on outer query and run by its own.2.In case of correlated subquery, outer query executed before inner query or subquery while in case of NonCorrelated subquery inner query executes before outer query.3.Correlated Sub-queries are slower than non correlated subquery and should be avoided in favor of sql joins.4.Common example of correlated subquery is using exits and not exists keyword while non correlated query mostly use IN or NOT IN keywords.

SubQuery vs Join in SQLAny information which you retrieve from database using subquery can be retrieved by using different types os joins also. Since SQL is flexible and it provides different way of doing same thing. Some people find SQL Joins confusing and subquery specially noncorrelated more intuitive but in terms of performance SQL Joins are more efficient than subqueries.

Important points about SubQuery in DBMS1.Almost whatever you want to do with subquery can also be done using join, it just matter of choicesubquery seems more intuitive to many user.2.Subquery normally return an scaler value as result or result from one column if used along withIN Clause.3.You can use subqueries in four places: subquery as a column in select clause,4.In case of correlated subquery outer query gets processed before inner query.

Difference between Primary key vs Foreign key in table SQL database tutorialMain difference between Primary key and Foreign key in a table is that, its the same column which behaves as primary key in parent table and as foreign key in child table. For example inCustomerandOrderrelationship,customer_idis primary key inCustomertable but foreign key inOrdertable. By the way what is foreign key in a table and difference between Primary and Foreign key are some of the popular SQL interview questions, much liketruncate vs delete in SQLordifference between correlated and noncorrelated subquery. We have been learning key SQL concepts along with these frequently asked SQL questions and in this SQL tutorial we will discuss about what is foreign key in SQL and purpose of foreign key in any table. By the way this is the third article related to primary key in SQL, other beingdifference between primary and unique keyandHow to find second highest salary in SQL. If you are preparing for any technical job interview where you expect some SQL questions, check out these questions, they are worth preparing.

What is Foreign key in a tableForeign key is a column in one table which is primary key on another table. Foreign key and Primary key is used to define relationship between two tables in relational database. For example in Employee and Department relationship, we have two tablesDepartment(dept_id, dept_name)andEmployee (emp_id, emp_name, dept_id).dept_idisprimary keyinDepartmenttable and foreign key inEmployeetable. Though its not require that name of foreign key must be same with primary key, we have kept it same as per standard SQL best practices. Foreign key in a table enforceReferential Integrity constraint, which can be used to implement business rules e.g. referential integrity can stop you from creating anEmployeewith a non existent department. This kind of check maintains integrity of data in a relationship. As discussed in our postWhat is referential integrity in MySQL database, we have seen that it's implemented as foreign key constraint and can allowCASCADE UPDATEandDELETE. These referential action delete or update matching column in child table ( foreign key table) when corresponding row from parent table (primary key table ) is deleted or updated to maintain integrity of data.

Difference between Primary key and Foreign key in SQLHere are some important difference between primary and foreign keys in a table which is worth remembering both on SQL interview point of view and knowledge point of view.

1) Name offoreign keycan be different than the name ofprimary keyit represent in other table. For example in ourEmployeeandDepartmentrelationship, Primary key inDepartmenttable isdept_idand we have used same name inEmployeetable to create foreign key. It could have been different e.g.departmentId or departmentIDt etc.

2) Another difference between primary and foreign key is that unlike primary key,foreign key can be nulle.g. in our example you can have anEmployeerecord for whichdept_idcan be null, this shows that no corresponding record inDepartmenttable.

3) One moredifference between primary key and foreign keyis thatforeign key can be duplicateopposite to primary key which is always unique.

4) By using foreign key constraints, we can introduce referential integrity in multiple table relationship in SQL. Referential integrity guarantees data integrity, seebenefits of Referential Integrity in SQLto know more.

5) Foreign key mostly work as link between two table when we join tables usingINNER JOINandOUTER JOIN. For example when weINNERJOINbothEmployeewithDepartmenttable, we can usedept_idas joining column. SeeHow to join three tables in SQLfor more details.

6) Table on which a column is declared asprimary keyis known as parent table in relationship andforeign keytable is known as child table in relationship. For example inEmployeeandDepartmentrelationship,Departmentis parent table becausedept_idis primary key there andEmployeeis child table becausedept_idis foreign key in this table.

Primary key and Foreign key Example in SQLOne of the best example to understand Primary key and Foreign key in a table isEmployeeandDepartmentrelationship orCustomerandOrderrelationship. You can createOrderandCustomertable in MySQL as following to create primary and foreign keys :

CREATETABLECustomer(cust_id INTNOTNULL, cust_name VARCHAR(256), PRIMARYKEY(cust_id))ENGINE=INNODB;

CREATETABLEORDER(order_id INTNOTNULL, amount INTNOTNULL, cust_id INT, FOREIGNKEY(cust_id)REFERENCESCustomer(cust_id) ONDELETECASCADE)ENGINE=INNODB;

Nowcust_idis primary key inCustomertable and foreign key inOrdertable. If we try to insert anOrderfor whichcust_idis something which is invalid inCustomertable, MySQL database will reject such INSERT or UPDATE. This is one of the benefit of using Referential Integrity. It also allow toCASCADE UPDATEandDELETEoperation which first delete or update a row in parent table e.g.Customerand then delete or update all matching rows in child table e.g.Ordertable.

That's all on what is foreign key in a table and difference between primary and foreign key in SQL. I suggest to create some table by yourself and try to test foreign key constraint by violating it and see how database e.g. Oracle, MySQL or SQL Serverbehaves. To understand more tryON DELETE CASCADEandON DELETE UPDATEto see how database maintains foreign key constraint. You can also see my post onReferential Integrity example on MySQL database

Difference between primary key vs Candidate Key in table - SQL databasePrimary key vs Candidate KeyWhat isdifference between primary key and candidate keyis another popular SQL and database interview questions which appears in variousprogramming interviewsnow and then. Concept of primary key and candidate key is not just important from interview point of view but also on designing database and normalization. By the way this is my second post in primary key, In last one we have seen comparison ofprimary key vs unique key, which is also happens to be one of the frequently asked database question. By definition primary key is a column or collection of columns, which uniquely define a row in a table. Candidate keys are keys which can be primary key and also able to uniquely identify any row in table. In simply terms you may have couple of Candidate keys and you have choose one of them as primary key. This selection part is the most important skill in database design. Since only primary key can have clustered index in table while unique keys can have Nonclustered index, its important to choose right column or collection of columns as primary key. Often I select a column which is most frequently used in Where clause of SELECT query. If you preparing for SQL interview or looking for some good SQL interview question than you can also checkdifference between Correlated and Noncorrelated subqueriesandWhen to use truncate vs delete in SQL.

Difference between Candidate Key vs Primary KeyBefore seeing difference between Candidate key and Primary key let's see some similarities between them in bullets points.

1) Both Primary and Candidate keys can uniquely identify records in a table on database.

2) Both Primary and Candidate keys are has constraints UNIQUE and NOT NULL.

3) Primary key or Candidate keys can be either single column or combination of multiple columns in a table.

Now from interview point of view here is difference between Candidate key and primary key in SQL table on point format for easy to remember :

1) There can be multiple Candidate keys in a table in relation database e.g.Oracle,MySQL, Sybase or MSSQL but only one primary key is permitted.

2) An example of Primary key and Candidate key can be ID and SSN number in a Employee table, Since both can identify each employee uniquely they are candidate key and any one can become primary key. Now if you have to choose between them as primary key, I will go ID asprimary keybecause SSN is sensitive information and may not be allow/not safe to use as String in queries as frequently as ID. Second reason of choosing ID over SSN as primary key can be use of ID as primary tracking ID within organization and its frequent use all over the place. Once you choose a primary key, All candidate keyare like unique keys.

That's all on difference between Primary key and Candidate key in a table. If you understand election well than you can think primary key as elected member among all candidate keys.

Question 1: SQL Query to find second highest salary of Employee

Answer : There are many ways to find second highest salary of Employee in SQL, you can either use SQL Join or Subquery to solve this problem. Here is SQL query using Subquery :

select MAX(Salary) from Employee WHERE Salary NOT IN (select MAX(Salary) from Employee );

SeeHow to find second highest salary in SQLfor more ways to solve this problem.

Question 2: SQL Query to find Max Salary from each department.

Answer :

SELECT DeptID, MAX(Salary) FROM Employee GROUP BY DeptID.

Question 3:Write SQL Query to display current date.

Ans:SQL has built in function calledGetDate()which returns current timestamp.

SELECT GetDate();

Question 4:Write an SQL Query to check whether date passed to Query is date of given format or not.

Ans: SQL hasIsDate()function which is used to check passed value is date or not of specified format ,it returns 1(true) or 0(false) accordingly.

SELECT ISDATE('1/08/13') AS "MM/DD/YY";

It will return 0 because passed date is not in correct format.

Question 5: Write a SQL Query to print the name of distinct employee whose DOB is between 01/01/1960 to 31/12/1975.

Ans:SELECT DISTINCT EmpName FROM Employees WHERE DOB BETWEEN 01/01/1960 AND 31/12/1975;

Question 6:Write an SQL Query find number of employees according to genderwhose DOB is between 01/01/1960 to 31/12/1975.

Answer :SELECT COUNT(*), sex from EmployeesWHEREDOB BETWEEN 01/01/1960 ' AND 31/12/1975GROUP BY sex;

Question 7:Write an SQL Query to find employee whose Salary is equal or greater than 10000.

Answer :SELECT EmpName FROMEmployees WHERESalary>=10000;

Question 8:Write an SQL Query to find name of employee whose name Start with M

Ans: SELECT * FROM Employees WHERE EmpName like 'M%';

Question 9: find all Employee records containing the word "Joe", regardless of whether it was stored as JOE, Joe, or joe.

Answer :SELECT* from Employees WHEREupper(EmpName) like upper('joe%');

Question 10: Write a SQL Query to findyear from date.

Answer : SELECT YEAR(GETDATE()) as "Year";

Difference between Self and Equi Join in SQL - INNER Join example MySQLMain difference between Self Join and Equi Join is that, In Self Join we join one table to itself rather than joining two tables. Both Self Join and Equi Join are types of INNER Join in SQL but there is subtle difference between two. Any INNER Join with equal as join predicate is known as Equi Join. SQL Joins are fundamental concept of SQL similar tocorrelated and noncorrelated subqueriesor using group by clause and good understanding of various types of SQL join is must for any programmer. By the way If you have written INNER join using where clause than using comparison operator as=will be known as equi join. Equi join or Self join are not a formal join or part of syntax, instead they are just popular way to refer certain join examples. One of the best example of Self Join, I have seen in SQL Interview questions is "How do you find all Employees who are Managers in Employee table", which is commonly asked along with another popular questionhow to find second highest salary of employeeor questions related tojoin three tables in one sql query. In this SQL tutorial we will learn self join by example while solving this SQL query.

Self Join Example in SQLIn order to solve this query let's first see schema and data ofEmployeetable.

mysql>select*fromemployee;+--------+----------+---------+--------+--------+|emp_id|emp_name|dept_id|salary|mgr_id|+--------+----------+---------+--------+--------+| 103|Jack | 2|1400| 104|| 104|John | 2|1450| 104|| 105|Johnny | 3|1050| 104|| 108|Alan | 3|1150| 104|| 106|Virat | 4| 850| 105|| 107|Vina | 4| 700| 105|| 109|joya | 4| 700| 105|+--------+----------+---------+--------+--------+7rowsinset(0.00sec)

In abovetable all employees who are managers has thereemp_idasmgr_idin other employees and by using SELF JOINi.e. join two instance of employee table and comparing, we can find all employees who are managers. Here is theSELECT query exampleusing self join :

mysql>selectdistincte.emp_id,e.emp_namefromemployee ejoinemployee mone.emp_id=m.mgr_id;+--------+----------+|emp_id|emp_name|+--------+----------+| 104|John || 105|Johnny |+--------+----------+2rowsinset(0.00sec)

In this example of Self Join, we have joined employee table to itself by using two table aliases e and m. We have also useddistinctkeyword to remove duplicates here. You can also say this is an example of EQUI JOIN because in join predicate we have used = or equal condition. In fact this one is example of INNER Join, SELF Join and EQUI Join at same time.

Self Join vs Equi JoinIn short major difference between Self Join and Equi Join in SQL is that Self Join requires only one table while most of Equi join is condition used in join predicate. Since Equi Join is based on condition for comparison, it can occur in any INNER, OUTER or SELF join in SQL.

Thats all on difference between Self Join and Equi Join in SQL. Self Join is one of the important technique to solve many SQL query related problem where two columns of table contains same type of data e.g. here emp_id and dept_id are essentially same data.