Adding a Foreign key!

 Hello everyone,

  I know that a Primary Key can be added to a table after it has been created, doing this...

CREATE UNIQUE INDEX table_pkey ON table (atribute);

  Well I wonder if a Foreign Key can also be added after the table is already created, and what syntax does it follow.

  Thankyou in advance.

  Stay Safe & Happy.

   :* TankGirl