A '''check constraint''' is applied to2 each [[Row (database)|row]] in the [[Table (database)|table]].{{Clarify|April 2013|date=April 2013}} The constraint must be a [[Predicate (mathematical logic)|predicate]]. It can refer to a single or multiple [[Column (database)|columns]] of the table. The result of the predicate can be either <code>TRUE</code>, <code>FALSE</code>, or <code>UNKNOWN</code>, depending on the presence of [[Null (SQL)|NULLs]]. If the predicate evaluates to <code>UNKNOWN</code>, then the constraint is not violated and the row can be inserted or updated in the table. This is contrary to predicates in [[Where (SQL)|WHERE]] clauses in [[Select (SQL)|SELECT]] or [[Update (SQL)|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:
