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SQL: DDL, DML and DCL

Data Definition Language

The Data Definition Language (DDL) manages table and index structure. The most basic items of DDL are the CREATE, ALTER, RENAME, DROP and TRUNCATE statements.

When defining a table, ensure that the SQL you use is as accurate as possible, in particular:

• Use the most appropriate data type
• Define primary, unique and foreign keys
• State whether a column can be NULL or NOT NULL
• Include any check constraints or default values

For performance reasons, data warehouse appliances often don't enforce referential integrity (although they will record the definitions); however to aid documentation it is useful to record the relationships in the DDL.

Data Manipulation Language

There are four basic Data Manipulation Language (DML) operations: SELECT, INSERT, UPDATE and DELETE.

Data Control Language

The Data Control Language (DCL) authorizes users and groups of users to access and manipulate data. Its two main statements are GRANT and REVOKE.