SQL Commands 

SQL commands are instructions used to communicate with the database to perform specific task that work with data. SQL commands can be used not only for searching the database but also to perform various other functions like, for example, you can create tables, add data to tables, or modify data, drop the table, set permissions for users. SQL commands are grouped into four major categories depending on their functionality:

• Data Definition Language (DDL) - These SQL commands are used for creating, modifying, and dropping the structure of database objects. The commands are CREATE, ALTER, DROP, RENAME, and TRUNCATE.
• Data Manipulation Language (DML) – These SQL commands are used for storing, retrieving, modifying, and deleting data. These commands are SELECT, INSERT, UPDATE, and DELETE.
• Transaction Control Language (TCL) – These SQL commands are used for managing changes affecting the data. These commands are COMMIT, ROLLBACK, and SAVEPOINT.
• Data Control Language (DCL) - These SQL commands are used for providing security to database objects. These commands are GRANT and REVOKE.

Commercial Databases

• Oracle

Oracle is the most popular relational database. It runs on both Unix and Windows. It used to be many times more expensive than SQL Server and DB2, but it has come down a lot in price.

• SQL Server

SQL Server is Microsoft’s database and, not surprisingly, only runs on Windows. It has only a slightly higher market share than Oracle on Windows machines. Many people find it easier to use than Oracle.

• DB2

IBM’s DB2 was one of the earliest players in the database market. It is still very commonly used on mainframes and runs on both Windows and Unix.

Popular Open Source Databases

• MySQL

Because of its small size, its speediness, and its very good documentation, MySQL has quickly become the most popular open source database. MySQL is available on both Windows and Unix, but it lacks some key features such as support for stored procedures.

• PostgreSQL

Until recently, PostgreSQL was the most popular open source database until that spot was taken over by MySQL. PostgreSQL now calls itself “the world’s most advanced Open Source database software.” It is certainly a featureful and robust database management system and a good choice for people who want some of the advanced features that MySQL doesn’t yet have. PostgreSQL does not yet natively support Windows, but it is supposed to in the upcoming 7.5 release.