A check constraint (also known as table check constraint) is a condition that defines valid data when adding or updating an entry in a table of a relational database. A check constraint is applied to each row in the table. The constraint must be a predicate. It can refer to a single 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:
PRICE >= 0 QUANTITY >= 0If 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.
Read more about Check Constraint: Definition, NOT NULL Constraint, Common Restrictions
Famous quotes containing the words check and/or constraint:
“cried as he died, fearing at last the spheres’
Last sound, the world going out without a breath:
Too proud to cry, too frail to check the tears,
And caught between two nights, blindness and death.”
—Dylan Thomas (1914–1953)
“In America a woman loses her independence for ever in the bonds of matrimony. While there is less constraint on girls there than anywhere else, a wife submits to stricter obligations. For the former, her father’s house is a home of freedom and pleasure; for the latter, her husband’s is almost a cloister.”
—Alexis de Tocqueville (1805–1859)