如果是兩個表之間的連接查詢：

select film_id from film_actor where actor_id in (select actor_id from actor where first_name = 'sandra')

select film_id from film_actor join actor on (film_actor.actor_id = actor.actor_id and actor.first_name = 'sandra')

這里，join查詢性能優(yōu)于子查詢，?比較好理解。性能如下：

子查詢：? ?Query_time: 0.008001? Lock_time: 0.000000 Rows_sent: 56? Rows_examined: 5462

join查詢： Query_time: 0.000996? Lock_time: 0.000996 Rows_sent: 56? Rows_examined: 256

但3個表的子查詢：

課件中的子查詢：

select title, release_year, length from film where film_id in(select film_id from film_actor where actor_id in (select actor_id from actor where first_name = 'sandra'));

我自己寫的連接查詢：

select title, release_year, length from film join film_actor join actor on (film.film_id = film_actor.film_id and film_actor.actor_id = actor.actor_id and actor.first_name = 'sandra');

執(zhí)行日志：

子查詢：? ? Query_time: 0.005999? Lock_time: 0.000000 Rows_sent: 56? Rows_examined: 1000

join查詢： Query_time: 0.008032? Lock_time: 0.000000 Rows_sent: 56? Rows_examined: 11924

請問，為什么這里，子查詢的性能要優(yōu)于連接查詢？