SQL CONSTRAINT Keyword

ADD CONSTRAINT

The ADD CONSTRAINT command is used to create a constraint after a table is already created.

The following SQL adds a constraint named "PK_Person" that is a PRIMARY KEY constraint on multiple columns (ID and LastName):

Example

  ALTER TABLE Persons
ADD CONSTRAINT PK_Person PRIMARY KEY (ID,LastName);

DROP CONSTRAINT

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

DROP a UNIQUE Constraint

To drop a UNIQUE constraint, use the following SQL:

ALTER TABLE Persons

DROP CONSTRAINT UC_Person;

ALTER TABLE Persons

DROP INDEX UC_Person;

SQL Server / Oracle / MS Access:

MySQL:

DROP a PRIMARY KEY Constraint

To drop a PRIMARY KEY constraint, use the following SQL:

ALTER TABLE Persons

DROP CONSTRAINT PK_Person;

ALTER TABLE Persons

DROP PRIMARY KEY;

SQL Server / Oracle / MS Access:

MySQL:

DROP a FOREIGN KEY Constraint

To drop a FOREIGN KEY constraint, use the following SQL:

ALTER TABLE Orders

DROP CONSTRAINT FK_PersonOrder;

ALTER TABLE Orders

DROP FOREIGN KEY FK_PersonOrder;

SQL Server / Oracle / MS Access:

MySQL:

DROP a CHECK Constraint

To drop a CHECK constraint, use the following SQL:

ALTER TABLE Persons

DROP CONSTRAINT CHK_PersonAge;

ALTER TABLE Persons

DROP CHECK CHK_PersonAge;

SQL Server / Oracle / MS Access:

MySQL: