http://www.tutorialspoint.com/sql/sql-check.htm 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 into the table. Example: For example, the following SQL creates a new table called CUSTOMERS and adds five columns. Here we add a CHECK with AGE column, so that you can not have any CUSTOMER 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 CUSTOMERS table has already been created, then to add a CHECK constraint to AGE column, you would write a statement similar to the following: ALTER TABLE CUSTOMERS ADD CONSTRAINT myCheckConstraint CHECK(AGE >= 18); DROP a CHECK Constraint: To drop a CHECK constraint, use the following SQL. This syntax does not work with MySQL: ALTER TABLE CUSTOMERS DROP CONSTRAINT myCheckConstraint;
