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explain select *,
(select count(*) from sakila.film as outer_sub where outer_sub.film_id = outer_der.film_id)
from (
select *,
(select count(*) from sakila.film as mid_sub where mid_sub.film_id = mid_der.film_id)
from (
select *,
(select count(*) from sakila.film as inner_sub where inner_sub.film_id = inner_der.film_id)
from sakila.film as inner_der
) as mid_der
) as outer_der;
+----+--------------------+------------+--------+---------------+---------+---------+-------------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------+--------+---------------+---------+---------+-------------------+------+--------------------------+
| 1 | PRIMARY | <derived3> | ALL | NULL | NULL | NULL | NULL | 1000 | |
| 3 | DERIVED | <derived5> | ALL | NULL | NULL | NULL | NULL | 1000 | |
| 5 | DERIVED | inner_der | ALL | NULL | NULL | NULL | NULL | 951 | |
| 6 | DEPENDENT SUBQUERY | inner_sub | eq_ref | PRIMARY | PRIMARY | 2 | inner_der.film_id | 1 | Using where; Using index |
| 4 | DEPENDENT SUBQUERY | mid_sub | eq_ref | PRIMARY | PRIMARY | 2 | mid_der.film_id | 1 | Using where; Using index |
| 2 | DEPENDENT SUBQUERY | outer_sub | eq_ref | PRIMARY | PRIMARY | 2 | outer_der.film_id | 1 | Using where; Using index |
+----+--------------------+------------+--------+---------------+---------+---------+-------------------+------+--------------------------+
|
