Domain Constraints (SQL)

We can also specify domain constraints using CHECK by providing a valid conditional expression:

DNUMBER INT CHECK (DNUMBER > 0 AND DNUMBER < 21);

We can also use Create Domain (SQL) to specify checks:

CREATE DOMAIN D_NUM AS INTEGER CHECK (VALUE > 0 AND VALUE < 21);

Check is unable to compare against other attributes / relations, in those cases we use an ASSERTION.