Usage and examples of SQL CONSTRAINT keyword

SQL Keyword Reference

ADD CONSTRAINT

The ADD CONSTRAINT command is used to create constraints after the table is created.

The following SQL adds a constraint named 'PK_Person' on multiple columns (ID and LastName):

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

DROP CONSTRAINT

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

Delete UNIQUE constraint

To delete the UNIQUE constraint, please use the following SQL:

SQL Server / Oracle / MS Access：

ALTER TABLE Persons
DROP CONSTRAINT UC_Person;

MySQL：

ALTER TABLE Persons
DROP INDEX UC_Person;

Delete PRIMARY constraint

To delete the PRIMARY KEY constraint, please use the following SQL:

SQL Server / Oracle / MS Access：

ALTER TABLE Persons
DROP CONSTRAINT PK_Person;

MySQL：

ALTER TABLE Persons
DROP PRIMARY KEY;

Delete FOREIGN KEY constraint

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

SQL Server / Oracle / MS Access：

ALTER TABLE Orders
DROP CONSTRAINT FK_PersonOrder;

MySQL：

ALTER TABLE Orders
DROP FOREIGN KEY FK_PersonOrder;

Delete CHECK constraint

To delete a CHECK constraint, use the following SQL:

SQL Server / Oracle / MS Access：

ALTER TABLE Persons
DROP CONSTRAINT CHK_PersonAge;

MySQL：

ALTER TABLE Persons
DROP CHECK CHK_PersonAge;

SQL Keyword Reference