The CHECK constraint is used to limit the value range that can be placed in a column.
If you define a CHECK constraint on a single column it allows only certain values for this column. If you define a CHECK constraint on a table it can limit the values in certain columns based on values in other columns in the row.
SQL CHECK Constraint on CREATE TABLE
The following SQL creates a CHECK constraint on the “P_Id” column when the “Persons” table is created. The CHECK constraint specifies that the column “P_Id” must only include integers greater than 0.
My SQL:
CREATE TABLE Persons ( P_Id int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Address varchar(255), City varchar(255), CHECK (P_Id>0) )
SQL Server / Oracle / MS Access:
CREATE TABLE Persons ( P_Id int NOT NULL CHECK (P_Id>0), LastName varchar(255) NOT NULL, FirstName varchar(255), Address varchar(255), City varchar(255) )
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 ( P_Id int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Address varchar(255), City varchar(255), CONSTRAINT chk_Person CHECK (P_Id>0 AND City=’Sandnes’) )
SQL CHECK Constraint on ALTER TABLE
To create a CHECK constraint on the “P_Id” column when the table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Persons ADD CHECK (P_Id>0)
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_Person CHECK (P_Id>0 AND City=’Sandnes’)
To DROP a CHECK Constraint
To drop a CHECK constraint, use the following SQL:
SQL Server / Oracle / MS Access:
ALTER TABLE Persons DROP CONSTRAINT chk_Person
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