CHECK

The CHECK constraint restricts the permissible values that can be inserted into a column within a database table.

SQL CHECK on CREATE TABLE

The provided SQL establishes a CHECK constraint on the “Age” column during the creation of the “Persons” table. This constraint guarantees that no person under the age of 18 can be added.

MySQL:

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

SQL Server / Oracle / MS Access:

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

For the ability to assign a specific name to a CHECK constraint and to define such a constraint across multiple columns, adhere to 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

For creating a CHECK constraint on the “Age” column after the table has been created, utilize the following SQL:

MySQL / SQL Server / Oracle / MS Access:

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

For enabling the naming of a CHECK constraint and defining it across 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 remove a CHECK constraint, employ the following SQL:

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT CHK_PersonAge;

MySQL:

ALTER TABLE Persons
DROP CHECK CHK_PersonAge;