FOREIGN KEY

    - It is used to establish relationship between 2 tables.
    - Values of foreign key should match with values of primary key/ unique keys.
    - It allows NULL values, duplicate values.
    - A parent child relationship will be decalred when once the foreign key constraint is declared.
        This relationship is also called as 'Master/ Detail relationship', 'Master/ Transaction relationship' , 'Master/ Salve relationship'.
    - The table holding Primary Key is called Parent or Master.
    - The table holding Foreign Key is called Child or Details Or Transaction or Slave.
    - By default, Oracle creates one- many realtion ship for every parent-child relationship.
    - To establish one-to-one relationship, decalre foreign key with Unique constraints.
   
    Syntax:
        column_name datatype [size] references table_name ( column_name );
   
    Examples:

    (i) Creating a Check Constraint.
       
        Column Level Check declaration.
            create table emp
            (empno number(4), ..., deptno number(4) CONSTRAINT fk_deptno REFERENCES dept( deptno ), ........  );
       
        Table level Check Constraint.
            create table emp
            (empno number(4), ...... deptno number(4), .......,
             foreign key (deptno) CONSTRAINT fk_deptno REFERENCES dept( deptno ),...............);
       
        Using Alter
            create table emp
            (empno number(4), ...... deptno number(4), .......)

            Alter table emp add (  foreign key (deptno) CONSTRAINT fk_deptno REFERENCES dept( deptno ) );
   
    (ii) Drop Check constraint.
       
        Alter table emp drop CONSTRAINT fk_deptno;