ALTER TABLE unite_maintenance
   ADD COLUMN id_contrats integer;

update unite_maintenance um set 
id_contrats = (
select max(pre.id_contrats)
from prestations pre
inner join prestations_unite_maintenance pum on pre.id_prestations = pum.id_prestations
where pum.id_unite_maintenance=um.id_unite_maintenance 
)
;

--alter table prestations drop column id_contrats

--Delete from unite_maintenance where id_contrats = 696
--Delete from unite_maintenance where id_contrats = 690

ALTER TABLE unite_maintenance ADD FOREIGN KEY (id_contrats) REFERENCES contrats (id_contrats)
   ON UPDATE NO ACTION ON DELETE NO ACTION;
CREATE INDEX fki_unite_maintenance_contrat ON unite_maintenance(id_contrats);