mysql Flere fremmednøgler i en tabel til den samme primærnøgle

Nej, du skal oprette to fremmednøgler:

ADD CONSTRAINT `ufd_users_fk` FOREIGN KEY (`userId`) 
  REFERENCES `users` (`userId`) 
  ON DELETE CASCADE ON UPDATE CASCADE,
ADD CONSTRAINT `ufd_users_fk` FOREIGN KEY (`friendId`) 
  REFERENCES `users` (`userId`) 
  ON DELETE CASCADE ON UPDATE CASCADE;