[SQL] problem with uniques and foreing keys

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Hi list...

Please i have a problem with this...

I create the follow tables...

--------------------------------------------------------

CREATE TABLE empresa (
  id_empresa         integer  NOT NULL primary key,
  nombre             varchar(45),
);
CREATE TABLE casino (
  id_casino          integer  NOT NULL,
  id_empresa         integer      REFERENCES empresa(id_empresa),

  nombre             varchar(45),

  primary key(id_casino,id_empresa)
);
CREATE TABLE maq_casino (
  id_empresa      integer  NOT NULL REFERENCES  casino(id_empresa),
  id_casino       integer  NOT NULL REFERENCES  casino(id_casino),

  ubicacion_sala     varchar(45) default NULL,
  primary key(id_empresa,id_casino,id_tipo_maquina,id_maq_casino)
);

--------------------------------------------------------

When i'm gonna to create the last table i got this error:

ERROR:  no hay restriccion unique que coincida con las columnas dadas en la tabla referida <<casino>>

That in english is like .. there is no a unique constraint with columns referred  in casino table.


Please where is the problem...

Greetings ...

Krackem