Sql 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.

Scope

Constraints may be applied at the column-level or the table-level:

Column-level constraints

Declared as part of a column definition and apply only to that column.


Table-level constraints

Declared independently from any column definitions and may apply to one or more columns in the table. A table constraint is required when you wish to define a constraint that applies to more than one column.