rlcc885

UPDATE tabla1 t1 
SET t1.valor = (SELECT t2.CODE FROM tabla2 t2 WHERE t1.valor = t2.DESC and t1.fecha=t2.fecha)
WHERE t1.filtro='correcto'
  AND EXISTS (SELECT t2.CODE FROM tabla2 t2 WHERE t1.valor = t2.DESC and t1.fecha=t2.fecha);

-- O

UPDATE
  (SELECT t1.valor  as OLD, t2.CODE as NEW 
  FROM tabla1 t1