Check Constraint

A check constraint is a type of integrity constraint in SQL which specifies a requirement that must be met by each row in a database table. The constraint must be a predicate. It can refer to a single column, or multiple columns of the table. The result of the predicate can be either TRUE, FALSE, or UNKNOWN, depending on the presence of NULLs. If the predicate evaluates to UNKNOWN, then the constraint is not violated and the row can be inserted or updated in the table. This is contrary to predicates in WHERE clauses in SELECT or UPDATE statements.

For example, in a table containing products, one could add a check constraint such that the price of a product and quantity of a product is a non-negative value:

If these constraints were not in place, it would be possible to have a negative price (−$30) or quantity (−3 items).

Check constraints are used to ensure the validity of data in a database and to provide data integrity. If they are used at the database level, applications that use the database will not be able to add invalid data or modify valid data so the data becomes invalid, even if the application itself accepts invalid data.

Each check constraint has to be defined in the CREATE TABLE or ALTER TABLE statement using the syntax:

If the check constraint refers to a single column only, it is possible to specify the constraint as part of the column definition.

A NOT NULL constraint is functionally equivalent to the following check constraint with an IS NOT NULL predicate:

Some relational database management systems are able to optimize performance when the NOT NULL constraint syntax is used as opposed to the CHECK constraint syntax given above.

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