Structured Query Language (SQL) - Cultural View of Technology

Check Constraint 19
Check Constraint
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 >= 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 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.
Definition
Each check constraint has to be defined in the CREATE TABLE or ALTER TABLE statement using the syntax:
CREATE TABLE table_name (
)
...,
CONSTRAINT constraint_name CHECK ( predicate ),
...
ALTER TABLE table_name
ADD CONSTRAINT constraint_name CHECK ( predicate )
If the check constraint refers to a single column only, it is possible to specify the constraint as part of the column
definition.
CREATE TABLE table_name (
)
...
column_name type CHECK ( predicate ),
...