Acknowledgement

This document is designed to aid Beginners to know more about database programming, guide, help and give tips on how DataBase works. Although the content of the document originated from W3 studio but you can learn more by cruising through their website (www.w3school.com)from more live practical, examples and help tips of SQL and other DataBase systems. Enjoy!

Ahmed Ibrahim Omeiza

SQL Tutorial

Introduction to SQL

SQL is a standard language for accessing and manipulating databases.

What is SQL?

• SQL stands for Structured Query Language
• SQL lets you access and manipulate databases
• SQL is an ANSI (American National Standards Institute) standard

What Can SQL do?

• SQL can execute queries against a database
• SQL can retrieve data from a database
• SQL can insert records in a database
• SQL can update records in a database
• SQL can delete records from a database
• SQL can create new databases
• SQL can create new tables in a database
• SQL can create stored procedures in a database
• SQL can create views in a database
• SQL can set permissions on tables, procedures, and views

SQL is a Standard - BUT....

Although SQL is an ANSI (American National Standards Institute) standard, there are many different versions of the SQL language.

However, to be compliant with the ANSI standard, they all support at least the major commands (such as SELECT, UPDATE, DELETE, INSERT, WHERE) in a similar manner.

Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!

Using SQL in Your Web Site

To build a web site that shows some data from a database, you will need the following:

• An RDBMS database program (i.e. MS Access, SQL Server, MySQL)
• A server-side scripting language, like PHP or ASP
• SQL
• HTML / CSS

RDBMS

RDBMS stands for Relational Database Management System.

RDBMS is the basis for SQL, and for all modern database systems like MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access.

The data in RDBMS is stored in database objects called tables.

A table is a collections of related data entries and it consists of columns and rows.

SQL Syntax

Database Tables

A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data.

Below is an example of a table called "Persons":

The table above contains three records (one for each person) and five columns (P_Id, LastName, FirstName, Address, and City).

SQL Statements

Most of the actions you need to perform on a database are done with SQL statements.

The following SQL statement will select all the records in the "Persons" table:

In this tutorial we will teach you all about the different SQL statements.

Keep in Mind That...

• SQL is not case sensitive
•  

Semicolon after SQL Statements?

Some database systems require a semicolon at the end of each SQL statement.

Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.

We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but some database programs force you to use it.

SQL DML and DDL

SQL can be divided into two parts: The Data Manipulation Language (DML) and the Data Definition Language (DDL).

The query and update commands form the DML part of SQL:

• SELECT - extracts data from a database
• UPDATE - updates data in a database
• DELETE - deletes data from a database
• INSERT INTO - inserts new data into a database

The DDL part of SQL permits database tables to be created or deleted. It also define indexes (keys), specify links between tables, and impose constraints between tables. The most important DDL statements in SQL are:

• CREATE DATABASE - creates a new database
• ALTER DATABASE - modifies a database
• CREATE TABLE - creates a new table
• ALTER TABLE - modifies a table
• DROP TABLE - deletes a table
• CREATE INDEX - creates an index (search key)
• DROP INDEX - deletes an index

SQL SELECT Statement

This chapter will explain the SELECT and the SELECT * statements.

The SQL SELECT Statement

The SELECT statement is used to select data from a database.

The result is stored in a result table, called the result-set.

SQL SELECT Syntax

and

Note: SQL is not case sensitive. SELECT is the same as select.

An SQL SELECT Example

The "Persons" table:

Now we want to select the content of the columns named "LastName" and "FirstName" from the table above.

We use the following SELECT statement:

The result-set will look like this:

SELECT * Example

Now we want to select all the columns from the "Persons" table.

We use the following SELECT statement: 

Tip: The asterisk (*) is a quick way of selecting all columns!

The result-set will look like this:

Navigation in a Result-set

Most database software systems allow navigation in the result-set with programming functions, like: Move-To-First-Record, Get-Record-Content, Move-To-Next-Record, etc.

Programming functions like these are not a part of this tutorial. To learn about accessing data with function calls, please visit our ADO tutorial or our PHP tutorial.

SQL SELECT DISTINCT Statement

This chapter will explain the SELECT DISTINCT statement.

The SQL SELECT DISTINCT Statement

In a table, some of the columns may contain duplicate values. This is not a problem, however, sometimes you will want to list only the different (distinct) values in a table.

The DISTINCT keyword can be used to return only distinct (different) values.

SQL SELECT DISTINCT Syntax

SELECT DISTINCT Example

The "Persons" table:

Now we want to select only the distinct values from the column named "City" from the table above.

We use the following SELECT statement:

The result-set will look like this:

SQL WHERE Clause

The WHERE clause is used to filter records.

The WHERE Clause 

The WHERE clause is used to extract only those records that fulfill a specified criterion.

SQL WHERE Syntax

WHERE Clause Example

The "Persons" table:

Now we want to select only the persons living in the city "Sandnes" from the table above.

We use the following SELECT statement:

The result-set will look like this:

Quotes Around Text Fields

SQL uses single quotes around text values (most database systems will also accept double quotes).

Although, numeric values should not be enclosed in quotes.

For text values:

For numeric values:

Operators Allowed in the WHERE Clause

With the WHERE clause, the following operators can be used:

Note: In some versions of SQL the <> operator may be written as !=

SQL AND & OR Operators

The AND & OR operators are used to filter records based on more than one condition.

The AND & OR Operators

The AND operator displays a record if both the first condition and the second condition is true.

The OR operator displays a record if either the first condition or the second condition is true.

AND Operator Example

The "Persons" table:

Now we want to select only the persons with the first name equal to "Tove" AND the last name equal to "Svendson":

We use the following SELECT statement:

The result-set will look like this:

OR Operator Example

Now we want to select only the persons with the first name equal to "Tove" OR the first name equal to "Ola":

We use the following SELECT statement:

The result-set will look like this:

Combining AND & OR

You can also combine AND and OR (use parenthesis to form complex expressions).

Now we want to select only the persons with the last name equal to "Svendson" AND the first name equal to "Tove" OR to "Ola":

We use the following SELECT statement:

The result-set will look like this:

SQL ORDER BY Keyword

The ORDER BY keyword is used to sort the result-set.

The ORDER BY Keyword

The ORDER BY keyword is used to sort the result-set by a specified column.

The ORDER BY keyword sort the records in ascending order by default.

If you want to sort the records in a descending order, you can use the DESC keyword.

SQL ORDER BY Syntax

ORDER BY Example

The "Persons" table:

Now we want to select all the persons from the table above, however, we want to sort the persons by their last name.

We use the following SELECT statement:

The result-set will look like this:

ORDER BY DESC Example

Now we want to select all the persons from the table above, however, we want to sort the persons descending by their last name.

We use the following SELECT statement:

The result-set will look like this:

SQL INSERT INTO Statement

The INSERT INTO statement is used to insert new records in a table.

The INSERT INTO Statement

The INSERT INTO statement is used to insert a new row in a table.

SQL INSERT INTO Syntax

It is possible to write the INSERT INTO statement in two forms.

The first form doesn't specify the column names where the data will be inserted, only their values:

The second form specifies both the column names and the values to be inserted:

SQL INSERT INTO Example

We have the following "Persons" table:

Now we want to insert a new row in the "Persons" table.

We use the following SQL statement:

The "Persons" table will now look like this:

Insert Data Only in Specified Columns

It is also possible to only add data in specific columns.

The following SQL statement will add a new row, but only add data in the "P_Id", "LastName" and the "FirstName" columns:

The "Persons" table will now look like this:

SQL UPDATE Statement

The UPDATE statement is used to update records in a table.

The UPDATE Statement

The UPDATE statement is used to update existing records in a table.

SQL UPDATE Syntax

Note: Notice the WHERE clause in the UPDATE syntax. The WHERE clause specifies which record or records that should be updated. If you omit the WHERE clause, all records will be updated!

SQL UPDATE Example

The "Persons" table:

Now we want to update the person "Tjessem, Jakob" in the "Persons" table.

We use the following SQL statement:

The "Persons" table will now look like this:

SQL UPDATE Warning

Be careful when updating records. If we had omitted the WHERE clause in the example above, like this:

The "Persons" table would have looked like this:

SQL DELETE Statement

The DELETE statement is used to delete records in a table.

The DELETE Statement

The DELETE statement is used to delete rows in a table.

SQL DELETE Syntax

Note: Notice the WHERE clause in the DELETE syntax. The WHERE clause specifies which record or records that should be deleted. If you omit the WHERE clause, all records will be deleted!

SQL DELETE Example

The "Persons" table:

Now we want to delete the person "Tjessem, Jakob" in the "Persons" table.

We use the following SQL statement:

The "Persons" table will now look like this:

Delete All Rows

It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:

Note: Be very careful when deleting records. You cannot undo this statement!