SQLite - 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.
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 COMPANY( ID INT PRIMARY KEY , NAME TEXT NOT NULL, AGE INT NOT NULL UNIQUE, ADDRESS CHAR (25) , SALARY REAL , );
If COMPANY 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 COMPANY MODIFY AGE INT NOT NULL CHECK (AGE >= 18 );
You can also use following syntax, which supports naming the constraint and multiple columns as well:
ALTER TABLE COMPANY ADD CONSTRAINT myCheckConstraint CHECK(AGE >= 18);