CHECK Constraint
CHECK Constraint
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
The following SQL creates a new table called CUSTOMERS. 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),
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
MODIFY AGE INT NOT NULL CHECK (AGE >= 18 );
Naming the constraint in multiple columns as well:
ALTER TABLE CUSTOMERS
ADD CONSTRAINT myCheckConstraint CHECK(AGE >= 18);
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