when foreign  key constraint is created on a column 
referential integrity is achieved .
when foreign key constraint is created on a column,that 
column reference other column in the different table or 
even in the same table but the reference column must be 
primary key or unique key.
when a new value is added in a column referenced with 
foreign key ,then the foreign key relation checks whether 
the new value matches with the reference column or not.
If value matches then they  are allowed into the table