IMCA - Introduction to RDBMS

A CHECK constraint  is used in SQL to limit the values that can be inserted or updated in a column  of a table. It ensures that all values in a column satisfy a specific condition. 

If the condition evaluates to TRUE , the value is accepted. If it's FALSE or UNKNOWN , the operation (like INSERT or UPDATE) fails.

Example :

CREATE TABLE Persons (
    ID INT PRIMARY KEY,
    Name VARCHAR(50),
    Age INT CHECK (Age >= 18)
);

INSERT INTO Persons (ID, Name, Age) VALUES (1, 'John', 20);