Structured Query Language

SQL: An Introduction

SQL (Pronounced S.Q.L)• The standard user and application program interface

to a relational database is the Structured Query Language (SQL)

• SQL statements are used both for interactive queries for information from a relational database and for gathering data for reports– SQL uses keywords to interact with databases– It allows creating a database, populating, and querying it– In this slide we learn to apply MySQL Workbench to do all

of these• You can also use MySQL client

Create a Database/Table

• The syntax to create a database : CREATE DATABASE databaseName //Comment: SQL is case sensitive!

• The CREATE keyword is also used to create a tableCREATE TABLE tablename //Capitalize keywords for clarity!(

Column1 description, //define each column

Column2 description, // put a comma after each

…)

Note: text written after // is comment, and is NOT part of the syntax!

Use MySQL to name the database

• Connect to MySQL // Read Chapter 4 Ullman– Make sure you have permission to create database

CREATE DATABASE sitename;// end each statement with a ;

USE sitename;// tells MySQL that you want to use the database from here on

Create the users Table in the DatabaseCREATE TABLE user(User_id MEDIUMINT UNSIGNED NOT NULL AUTO_INCREMENT,first_name VARCHAR(20) NOT NULL,Last_name VARCHAR(40) NOT NULL,email VARCHAR(60) NOT NULL,pass CHAR(40) NOT NULL,registration_date DATETIME NOT NULL,PRIMARY KEY (user_id), );

NOTE: The user table was designed in Chapter 4 Ullman’s book!

Confirm that the table exists

• SHOW TABLES;• SHOW CCOLUMNS FROM users;

Populating the table• Now that the table exists, insert records into it• For this purpose we use the INSERT syntax:• When we want only to enter values for certain columns:INSERT INTO tablename(column1, column2, …) VALUES (value1, value2, …);

// Columns not given a value will get NULL unless they have a default // value. If they are NOT NULL and have no default there will be an error

• When we want to enter values for all columns use the following:

INSERT INTO tablename VALUES (value1, NULL, value2, value3, …)

Inserting multiple rows (records)INSERT INTO tablename (column1, column2) VALUES (valueA, valueB), (valueC, valueD), (valueE, valueF),

// this is not supported by all database systems (only by MySQL)

Insert real data into the users tableINSERT INTO users(first_name, last_name, email, pass, registration_date,)VALUES (‘Hassan’, ‘Babaie’, ‘mylonite_7@yahoo.com’, SHA1(‘mypassword’), NOW());

//or without naming columnsINSERT INTO usersVALUES (NULL, ‘Hassan’, ‘Babaie’, ‘mylonite_7@yahoo.com’, SHA1(‘mypassword’), NOW()); // note: NULL is for the user_id (since all values must be provided!)

//Comments:// note: put strings in “quotation marks”!// char, varchar, and text are strings!// NULL should not be quoted// backslash escapes a mark, e.g., O’clock is written as ‘O\’clock’//the SHA1() and NOW() are functions.//the SHA1() encripts data to a 40 character long string, hence //CHAR(40) is used for password

Joins• Table joins are used to query data from two or

more tables, based on a relationship between certain columns in these tables

• Tables in a database are often related to each other with keysTable Store_Information

store_name Sales DateLos Angeles $1500 Jan-05-1999San Diego $250 Jan-07-1999Los Angeles $300 Jan-08-1999Boston $700 Jan-08-1999

Table Geography

Region_name store_nameEast BostonEast New YorkWest Los AngelesWest San Diego

• To get the sales information by region, we have to combine the information from the two tables

• These two tables are linked by the common field, "store_name“

• The following SQL will retrieve the sales data by the region.

SELECT A1.region_name REGION, SUM(A2.Sales) SALES FROM Geography A1 JOIN Store_Information A2 ON A1.store_name = A2.store_name GROUP BY A1.region_name

• Result:

REGION SALES

East $700

West $2050

Join

• JOIN: Return rows when there is at least one match in both tables

• LEFT JOIN: Return all rows from the left table, even if there are no matches in the right table

• RIGHT JOIN: Return all rows from the right table, even if there are no matches in the left table

• FULL JOIN: Return rows when there is a match in one of the tables

Views• A Database View is a subset of the database sorted and displayed in a

particular way

• For each view, you can control which columns are displayed, what order they are displayed in, how the data is sorted, and what types of records to display

• A view consists of rows and columns just like a table

• The difference between a view and a table is that views are definitions built on top of other tables (or views), and do not hold data themselves

• If a data item is changing in the underlying table, the same change is reflected in the view

• A view can be built on top of a single table or multiple tables. It can also be built on top of another view

View Customer Table• CREATE TABLE Customer

(First_Name varchar(100), Last_Name varchar(100), Address varchar(50), City varchar(30), Country varchar(25), Birth_Date date NOT NULL)

• Use the following syntax to create a view called V_Customer that contains only the First_Name, Last_Name, and Country columns from this table

CREATE VIEW V_CustomerAS SELECT First_Name, Last_Name, CountryFROM Customer

• Now we have a view called V_Customer with the following structure:

View V_Customer(First_Name char(100),Last_Name char(100),Country char(25))

Alter Table• Alter Table - Once a table is created in the database, you can

use the Alter statement to change the structure of the table

• For example:• Add a column • Drop a column • Change a column name • Change the data type for a column

ALTER table Customer add Gender char(1) ALTER table Customer drop Country

Drop Table

• Drop Table - Sometimes we may need to delete a table in the database

• This allows to get rid of tables not needed anymore and frees up database space

• Use the following command to delete the Customer table.

DROP TABLE Customer

Truncate Table• TRUNCATE TABLE - Sometimes we may need to delete all of the

data in a table

• One way of doing this is with DROP TABLE

• But what if we wish to simply get rid of the data but not the table itself?

• For this, we can use the TRUNCATE TABLE command

• Use the following command to delete all data in Customer table.

TRUNCATE TABLE Customer

Insert Into

• INSERT INTO Statement – Insert statement is used to add one or more rows to a table

INSERT INTO Customer values ('4','Adams','Joe') – adds one row

INSERT INTO Store_Information (store_name, Sales, Date)SELECT store_name, Sales, DateFROM Sales_InformationWHERE Year(Date) = 1998

Update

• UPDATE Statement - allows to update data in an existing table

UPDATE Store_InformationSET Sales = 2000WHERE store_name = "Los Angeles"AND Date = "Jan-08-1999"

Delete

• DELETE Statement – allows to remove rows from a table

DELETE FROM Store_InformationWHERE store_name = "Los Angeles"

Stored procedures

• A stored procedure is executable code that is associated with, and generally stored in, the database

• Stored procedures usually collect and customize common operations, like inserting a row into a table, gathering statistical information about usage patterns, or encapsulating complex business logic and calculations