Check constraint

A check constraint is a type of <a href="/facts/Integrity_constraint/g3XGbKIH">integrity constraint</a> in <a href="/facts/SQL/UjQLJcYm">SQL</a> which specifies a requirement that must be met by each <a href="/facts/Row_(database)/PIpv7c9e">row</a> in a database <a href="/facts/Table_(database)/5INvK1Vd">table</a>. The constraint must be a <a href="/facts/Predicate_(mathematical_logic)/5WazAvKF">predicate</a>. It can refer to a single column, or multiple <a href="/facts/Column_(database)/WB91xSVs">columns</a> of the table. The result of the predicate can be either TRUE, FALSE, or UNKNOWN, depending on the presence of <a href="/facts/Null_(SQL)/RBnXTe7k">NULLs</a>. 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 <a href="/facts/Select_(SQL)/cYnKkmFo">SELECT</a> or <a href="/facts/Update_(SQL)/IQz2IKpz">UPDATE</a> 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:

price >= 0

quantity >= 0

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 <a href="/facts/Data_validation/tFCchJTM">validity of data</a> in a database and to provide <a href="/facts/Data_integrity/g3XGbKIH">data integrity</a>. 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.

Check constraint open-in-new

Check constraint