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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);
constraints CHECK-constraint

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