Check Constraint:

It is used to restrict the range of values that can be inserted in a column. If a column contain check constraint then only a particular range of values can be inserted in that column.

Example:

CREATE TABLE Employee(
       Emp_ID   INT              NOT NULL,
       Emp_NAME VARCHAR (20)     NOT NULL,
       Emp_AGE  INT              NOT NULL CHECK (Emp_AGE >= 18),
       Emp_ADDRESS  CHAR (25) ,
       Emp_SALARY   DECIMAL (18, 2),       
       PRIMARY KEY (Emp_ID)
);

In this example we have put check constraint on Emp_Age column and we cannot insert Employee whose age is less than 18.

Adding Check Constraint using Alter Command:

We can easily add Check constraint  using Alter command.

ALTER TABLE Employee
   ADD CONSTRAINT myCheck CHECK (Emp_AGE >= 18 );

Drop a Check Constraint:

ALTER TABLE Employee
   DROP CONSTRAINT myCheck;