UPDATE SELECT всегда возвращает 0

UPDATE
  geo 
SET 
  region_id = (
    SELECT 
      towns.region_id 
    FROM 
      towns  
    WHERE 
      towns.uid=geo.dictionary_id
  ) 
WHERE 
    geo.deleted=0

UPDATE geo SET geo.region_id = ( SELECT towns.region_id FROM towns
WHERE towns.uid=geo.dictionary_id ) WHERE geo.deleted=0 так попробуй

UPDATE geo 
LEFT JOIN towns on towns.uid=geo.dictionary_id
SET geo.region_id = towns.region_id 
WHERE geo.deleted=0

Есть вот такой вариант:

UPDATE geo, towns
SET geo.region_id = towns.region_id
WHERE geo.deleted = 0 and geo.dictionary_id = towns.uid