This guide presents key SQL concepts and commands. The experienced user might use it as a quick SQL reference while the less experienced user might benefit from following the step-by step SQL Tutorial.
SQL stands for Structured Query Language and is a computer language that was developed specifically to interact with RDBMS (Relational Database Management Systems).
It can be subdivided into three subsets:
- DDL (Data Definition Language), which contains commands for the creation, modification and deletion of physical data structures (such as tables, columns indexes…).
- DML (Data Manipulation Language), which contains commands for the creation, update, deletion or retrieval of Data from physical data structures (in other words creating, updating or deleting rows in data tables).
- DCL (Data Control Language) which contains permission related commands (such as setting user privileges for a set of data structures).
Over time, several versions of the SQL standard were developed and published by ANSI, ANSI SQL 1986, SQL 1989, SQL 1992 (a.k.a SQL92 or SQL2), SQL 1999 (a.k.a SQL99 or SQL3) and SQL 2003.
Various vendors propose a number of RDBMS (some of which are open source or free) which conform to a greater or lesser extent to those standard. Minimum conformance criteria must be met for a vendor to claim its RDBMS is SQL ANSI compliant but vendor specific variations remain.
To practice with SQL one needs an RDBMS as well as a SQL client software to write the commands, submit them and display their results.