SQL is a standard computer language for accessing and manipulating databases.
SQL stands for Structured Query Language
SQL allows you to access a database
SQL is an ANSI standard computer language
SQL can execute queries against a database
SQL can retrieve data from a database
SQL can insert new records in a database
SQL can delete records from a database
SQL can update records in a database
SQL is easy to learn
SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MySQL, Oracle, MS SQL Server, Microsoft Access, MSDE, DB2, Sybase, Postgres, Informix , etc.
Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!
A database most often contains one or more tables. Each table is identified by a name (e.g. "Books" or "Authors"). Tables contain records (rows) with data.
Below is an example of a table called "Books":
|
Title |
Author |
Publisher |
Year |
|
الدورة الدموية |
إبن النفيس |
دار العلم |
1650 |
|
Java 2 |
L. Johnston |
Fast Press |
2002 |
|
Linux and Unix |
J. Sam |
Fast Press |
2004 |
|
Linux Distributions |
J. Sam |
Fast Press |
2003 |
|
Operating Systems |
M. Stone |
Coriolis |
2005 |
The table above contains three records (one for each book) and four columns (Title, Author, Publisher, and Year).
With SQL, we can query a database and have a result set returned.
A query like this:
|
SELECT Title, Author FROM Books; |
Gives a result set like this:
|
Title |
Author |
|
الدورة الدموية |
إبن النفيس |
|
Java 2 |
L. Johnston |
|
Linux and Unix |
J. Sam |
|
Linux Distributions |
J. Sam |
|
Operating Systems |
M. Stone |
Note: Some database systems require a semicolon at the end of the SQL statement.
SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records.
These query and update commands together form the Data Manipulation Language (DML) part of SQL:
SELECT - extracts data from a database table
UPDATE - updates data in a database table
DELETE - deletes data from a database table
INSERT INTO - inserts new data into a database table
The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables.
The most important DDL statements in SQL are:
CREATE TABLE - creates a new database table
ALTER TABLE - alters (changes) a database table
DROP TABLE - deletes a database table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index
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