cine poate sa-mi explice dece nu este corecta urmatoarea interogare?

UPDATE ANAPAT...buletin
SET id_prescriptii_pacient =
(
SELECT prescriptii_pacienti.id FROM ANAPAT...prescriptii_pacienti, ANAPAT...buletin
WHERE buletin.id_pacient = prescriptii_pacienti.id_pacient
)
WHERE EXISTS
(
SELECT buletin.id FROM anapat...buletin, anapat...prescriptii_pacienti
WHERE buletin.id_pacient=prescriptii_pacienti.id_pacient
)