SQL Constraints - Check Key Constraint

Check Key constraint is used to restrict the values in a column within the specified condition.

Syntax:

CREATE TABLE <Table Name> (

    <Column Name> data type CHECK(Condition)

);

Example:

CREATE TABLE Employees (

    ID int NOT NULL Primary Key IDENTITY(1,1),

    LastName varchar(255) NOT NULL UNIQUE,

    FirstName varchar(255),

    Age int CHECK(Age>17 and Age<61)

);

Now, it not possible to enter age less than 17 and greater than 60.

Using Alter command

ALTER TABLE Employees
ADD CHECK (Age>17 and Age<61);