Dropping Integrity Constraints

For Dropping an integrity constraint if the rule in which it enforces is no longer true or if the constraint is no longer required. Drop an integrity constraint using the ALTER TABLE command and the DROP clause. For instance the subsequent statements drop integrity constraints:

ALTER TABLE dept
DROP UNIQUE (dname, loc);

ALTER TABLE emp
DROP PRIMARY KEY,
DROP CONSTRAINT dept_fkey;
DROP TABLE emp CASCADE CONSTRAINTS;

The two Alter statements drop the primary and unique keys. The third statement drops the EMP table along with all the constraints.