SQL CHECK

SQL CHECK Constraint

The CHECK constraint restricts the range of values that can be inserted into a column.

When applied to a column, it enforces specific allowable values.

When applied to a table, it can restrict values in certain columns based on values in other columns within the same row.

SQL CHECK on CREATE TABLE

This SQL statement establishes a CHECK constraint on the “Age” column during the creation of the “Persons” table. This constraint guarantees that a person’s age must be 18 or higher.

MySQL:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    CHECK (Age>=18)
);

SQL Server / Oracle / MS Access:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int CHECK (Age>=18)
);

To enable the naming of a CHECK constraint and to define it across multiple columns, employ the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    City varchar(255),
    CONSTRAINT CHK_Person CHECK (Age>=18 AND City=‘Sandnes’)
);

SQL CHECK on ALTER TABLE

If you need to establish a CHECK constraint on the “Age” column after the table has already been created, utilize the following SQL statement:

MySQL / SQL Server / Oracle / MS Access:

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

To name a CHECK constraint and define it on multiple columns, utilize 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 statement:

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT CHK_PersonAge;

MySQL:

ALTER TABLE Persons
DROP CHECK CHK_PersonAge;