我很好奇为何EXISTS()应该比更快地执行IN()。

当比尔·卡尔文提出一个要点时，我正在回答一个问题。当您使用EXISTS()它时，它使用相关子查询（依赖子查询），而IN（）仅使用子查询。

解释显示，EXISTS并且NOT EXISTS两者都使用了一个依赖子查询，并且IN / NOT IN都只使用了一个子查询..所以我很好奇关联子查询如何比子查询更快？

我以前使用过EXISTS，它的执行速度比IN快，这就是我感到困惑的原因。

这是带有说明的SQLFIDDLE

EXPLAIN SELECT COUNT(t1.table1_id) 
FROM table1 t1 
WHERE EXISTS
(   SELECT 1 
    FROM table2 t2
    WHERE t2.table1_id <=> t1.table1_id
);

+-------+-----------------------+-----------+-------+---------------+-----------+--------+--------------------------+--------+------------------------------+
| ID    |   SELECT_TYPE         |   TABLE   | TYPE  | POSSIBLE_KEYS |   KEY     |KEY_LEN |  REF                     |   ROWS |  EXTRA                       |
+-------+-----------------------+-----------+-------+---------------+-----------+--------+--------------------------+--------+------------------------------+
|  1    |   PRIMARY             |   t1      | index | (null)        |   PRIMARY |   4    | (null)                   |   4    |  Using where; Using index    |
|  2    |   DEPENDENT SUBQUERY  |   t2      | REF   | table1_id     |  table1_id|   4    | db_9_15987.t1.table1_id  |   1    |  Using where; Using index    |
+-------+-----------------------+-----------+-------+---------------+-----------+--------+--------------------------+--------+------------------------------+

EXPLAIN SELECT COUNT(t1.table1_id) 
FROM table1 t1 
WHERE NOT EXISTS
(   SELECT 1 
    FROM table2 t2
    WHERE t2.table1_id = t1.table1_id
);
+-------+-----------------------+-----------+-------+---------------+-----------+--------+--------------------------+--------+------------------------------+
| ID    |   SELECT_TYPE         |   TABLE   | TYPE  | POSSIBLE_KEYS |   KEY     |KEY_LEN |  REF                     |   ROWS |  EXTRA                       |
+-------+-----------------------+-----------+-------+---------------+-----------+--------+--------------------------+--------+------------------------------+
|  1    |   PRIMARY             |   t1      | index | (null)        |   PRIMARY |   4    | (null)                   |   4    |  Using where; Using index    |
|  2    |   DEPENDENT SUBQUERY  |   t2      | ref   | table1_id     |  table1_id|   4    | db_9_15987.t1.table1_id  |   1    |  Using index                 |
+-------+-----------------------+-----------+-------+---------------+-----------+--------+--------------------------+--------+------------------------------+

EXPLAIN SELECT COUNT(t1.table1_id) 
FROM table1 t1 
WHERE t1.table1_id NOT IN 
(   SELECT t2.table1_id 
    FROM table2 t2
);
+-------+-------------------+-----------+-------+---------------+-----------+--------+----------+--------+------------------------------+
| ID    |   SELECT_TYPE     |   TABLE   | TYPE  | POSSIBLE_KEYS |   KEY     |KEY_LEN |  REF     |   ROWS |  EXTRA                       |
+-------+-------------------+-----------+-------+---------------+-----------+--------+----------+--------+------------------------------+
|  1    |   PRIMARY         |   t1      | index | (null)        |   PRIMARY |   4    | (null)   |   4    |  Using where; Using index    |
|  2    |   SUBQUERY        |   t2      | index | (null)        |  table1_id|   4    | (null)   |   2    |  Using index                 |
+-------+-------------------+-----------+-------+---------------+-----------+--------+----------+--------+------------------------------+

一些问题

在上面的解释，如何做EXISTS具有using where和using index在群众演员，但NOT EXISTS没有using where在演员？

相关子查询如何比子查询更快？