-- add a foreign key to abother table
alter table BOOKS
  add constraint BK_ATHR_FK foreign key (AUTHOR_ID)
  references AUTHORS (AUTHOR_ID);

-- add a foreign key to same table
-- PARENT_MENU_ID is a foreign key to primary key MENU_ID
alter table MENUS
  add constraint MENU_MENU_FK foreign key (PARENT_MENU_ID)
  references MENUS (MENU_ID);