SQL

CHECK Keyword

CHECK

The CHECK constraint limits the value that can be placed in a
column.

SQL CHECK on CREATE TABLE

The following SQL creates a CHECK constraint on the “Age” column when the “Persons” table is created.
The CHECK constraint ensures that you can not have any person below 18 years:

MySQL:

CREATE TABLE Persons
(
   
Age int,
   
CHECK (Age>=18)
);

SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
   
Age int CHECK (Age>=18)
);

To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons
(
   
Age int,
   
City varchar(255),
   
CONSTRAINT CHK_Person CHECK (Age>=18 AND City=’Sandnes’)
);

SQL CHECK on ALTER TABLE

To create a CHECK constraint on the “Age” column when the table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CHECK (Age>=18);

To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT CHK_PersonAge CHECK (Age>=18 AND City=’Sandnes’);

DROP a CHECK Constraint

To drop 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;