CHECK

    - It is used for applying Business rules on a paticular column.
        Like sal>2000, deptno is not null, lenght of username > 5, length of password >= 8 etc...
    - It allows NULL values.
       
    Syntax:
        column_name datatype [size] CONSTRAINT <constraint_name> CHECK <condition>;

    Examples:
   
    (i) Creating a Check Constraint.
       
        Column Level Check declaration.
            create table emp
            (empno number(4), ..., sal number(7,2) CONSTRAINT check_sal check (sal > 3000),............);
       
        Table level Check Constraint.
            create table emp
            (empno number(4), ...... sal number(7,2), .......,
             CONSTRAINT check_sal check (sal > 3000),...............);
       
        Using Alter
            create table emp
            (empno number(4), ...... sal number(7,2), .......)

            Alter table emp add (  CONSTRAINT check_sal check (sal > 3000) );
   
    (ii) Drop Check constraint.
       
        Alter table emp drop CONSTRAINT check_sal;