The CHECK Constraint enables a condition to check the value being entered into a record. If the condition evaluates to false, the record violates the constraint and isn't entered the table.
For example, the following program creates a new table called CUSTOMERS and adds five columns. Here, we add a CHECK with AGE column, so that you cannot have any CUSTOMER who is below 18 years.
CREATE TABLE CUSTOMERS( ID INT NOT NULL, NAME VARCHAR (20) NOT NULL, AGE INT NOT NULL CHECK (AGE >= 18), ADDRESS CHAR (25) , SALARY DECIMAL (18, 2), PRIMARY KEY (ID) );
If the CUSTOMERS table has already been created, then to add a CHECK constraint to AGE column, you would write a statement like the one given below.
ALTER TABLE CUSTOMERS MODIFY AGE INT NOT NULL CHECK (AGE >= 18 );
You can also use the following syntax, which supports naming the constraint in multiple columns as well −
ALTER TABLE CUSTOMERS ADD CONSTRAINT myCheckConstraint CHECK(AGE >= 18);
To drop a CHECK constraint, use the following SQL syntax. This syntax does not work with MySQL.
ALTER TABLE CUSTOMERS DROP CONSTRAINT myCheckConstraint;