Part of the
You Can Learn SQL series.
By Ken Brown
Updated: September 2, 2004
The Data Definition Language (DDL) is SQL statements that operate on the
relational database objects. This is the SQL tool that creates, changes and
destroys database tables, views, schemas and catalogs.
There are three commands available in the DDL that allow you to manipulate
The CREATE command allows you to create a new table or view.
The ALTER command is used to make changes to an existing database object
and modifies a table definition by altering, adding, or dropping columns and
constraints, or by disabling or enabling constraints and triggers.
The DROP command destroys or deletes the database object and removes a
table definition and all data, indexes, triggers, constraints, and permission
specifications for that table. Any view or stored procedure that references the
dropped table must be explicitly dropped by using the DROP VIEW or DROP
In review, the Data Definition Language is the SQL tool that creates,
changes and destroys database tables, views, schemas and catalogs. The commands
are CREATE, ALTER and DROP.