MySQL DROP CONSTRAINT Keyword

The MySQL DROP CONSTRAINT command is used to delete a UNIQUE, PRIMARY KEY, FOREIGN KEY, or CHECK constraint.

DROP a UNIQUE Constraint

To drop UC_Employee UNIQUE constraint from table called Employee, the below mentioned statement can be used:

ALTER TABLE Employee
DROP INDEX UC_Employee;

OR 

ALTER TABLE Employee
DROP CONSTRAINT UC_Employee;

DROP a PRIMARY KEY Constraint

To drop PK_Employee PRIMARY KEY constraint from table called Employee, the statement is given below:

ALTER TABLE Employee
DROP PRIMARY KEY;

DROP a FOREIGN KEY Constraint

To drop FK_Contact_Info FOREIGN KEY constraint from table called Contact_Info, the statement is given below:

ALTER TABLE Contact_Info
DROP FOREIGN KEY FK_Contact_Info;

OR

ALTER TABLE Contact_Info
DROP CONSTRAINT FK_Contact_Info;

DROP a CHECK Constraint

To drop CHK_Employee CHECK constraint from table called Employee, the statement is given below:

ALTER TABLE Employee
DROP CONSTRAINT CHK_Employee;

OR

ALTER TABLE Employee
DROP CHECK CHK_Employee;