exists與count(*)

使用count(*)

SELECT count(*)
FROM actor a
JOIN film_actor fa USING (actor_id)
WHERE a.last_name = 'WAHLBERG'      

執行計劃

使用exists

SELECT EXISTS (
  SELECT * FROM actor a
  JOIN film_actor fa USING (actor_id)
  WHERE a.last_name = 'WAHLBERG'
)      

執行計劃