You can attach CHECK constraints to domains. Columns defined on those domains inherit the CHECK constraints. A CHECK constraint explicitly specified for the column overrides that from the domain. For example, the CHECK clause in this domain definition requires that values inserted into columns only be positive integers.

CREATE DATATYPE positive_integer INT
CHECK ( @col > 0 );

Any column defined using the positive_integer domain accepts only positive integers unless the column itself has a CHECK constraint explicitly specified. Since any variable prefixed with the @ sign is replaced by the name of the column when the CHECK constraint is evaluated, any variable name prefixed with @ could be used instead of @col.

An ALTER TABLE statement with the DELETE CHECK clause drops all CHECK constraints from the table definition, including those inherited from domains.

Any changes made to a constraint in a domain definition (after a column is defined on that domain) are not applied to the column. The column gets the constraints from the domain when it is created, but there is no further connection between the two.

 See also