SQL in a Nutshell, 2nd Edition

Constraints

Constraints allow you to automatically enforce the integrity of data and to filter the data that is placed in a database. In a sense, constraints are rules that define which data values are valid during INSERT, UPDATE, and DELETE operations. When a data modification transaction breaks the rules of a constraint, the transaction is rejected.

In the ANSI standard, there are four constraint types: CHECK, PRIMARY KEY, UNIQUE, and FOREIGN KEY. (The RDBMS platforms may allow more. Refer to different platform sections of the CREATE/ALTER TABLE Statement to find out these exceptions.)