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MySQL的子查询及相关优化学习教程_MySQL

时间：2021-07-01 10:21:17 帮助过：14人阅读

一、子查询
1、where型子查询
（把内层查询结果当作外层查询的比较条件）

#不用order by 来查询最新的商品
select goods_id,goods_name from goods where goods_id = (select max(goods_id) from goods);

#取出每个栏目下最新的产品(goods_id唯一)
select cat_id,goods_id,goods_name from goods where goods_id in(select max(goods_id) from goods group by cat_id);

2、from型子查询
(把内层的查询结果供外层再次查询)
#用子查询查出挂科两门及以上的同学的平均成绩
思路：

#先查出哪些同学挂科两门以上
select name,count(*) as gk from stu where score < 60 having gk >=2;
#以上查询结果，我们只要名字就可以了，所以再取一次名字
select name from (select name,count(*) as gk from stu having gk >=2) as t;
#找出这些同学了，那么再计算他们的平均分
select name,avg(score) from stu where name in (select name from (select name,count(*) as gk from stu having gk >=2) as t) group by name;

3、exists型子查询
（把外层查询结果拿到内层，看内层的查询是否成立）

#查询哪些栏目下有商品，栏目表category,商品表goods
select cat_id,cat_name from category where exists(select * from goods where goods.cat_id = category.cat_id);

二、优化
从句式的形式看，子查询分为特殊格式子查询和非特殊格式子查询，特殊格式的子查询中又包括IN、ALL、ANY、SOME、EXISTS等类型的子查询，对于有的类型的子查询，MySQL有的支持优化，有的不支持，具体情况如下。

示例一，MySQL不支持对EXISTS类型的子查询的优化：

EXISTS类型的相关子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE EXISTS (SELECT 1 FROM t2 WHERE t1.a1= t2.a2 AND t2.a2>10);

+----+--------------------+-------+------+------+-------------+
| id | select_type    | table | type | key | Extra    |

+----+--------------------+-------+------+------+-------------+

| 1 | PRIMARY      | t1  | ALL | NULL | Using where |

| 2 | DEPENDENT SUBQUERY | t2  | ALL | NULL | Using where |

+----+--------------------+-------+------+------+-------------+

2 rows in set, 2 warnings (0.00 sec)

被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where exists(/* select#2 */

  select 1

  from `test`.`t2`

  where ((`test`.`t1`.`a1` = `test`.`t2`.`a2`) and (`test`.`t2`.`a2` > 10))

)

从查询执行计划看，子查询存在，MySQL没有进一步做子查询的优化工作。

另外的一个EXISTS类型的相关子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE EXISTS (SELECT 1 FROM t2 WHERE t1.b1= t2.b2 AND t1.a1=10);

+----+--------------------+-------+------+------+-------------+

| id | select_type    | table | type | key | Extra    |

+----+--------------------+-------+------+------+-------------+

| 1 | PRIMARY      | t1  | ALL | NULL | Using where |

| 2 | DEPENDENT SUBQUERY | t2  | ALL | NULL | Using where |

+----+--------------------+-------+------+------+-------------+

2 rows in set, 3 warnings (0.02 sec)

被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where exists(/* select#2 */

  select 1

  from `test`.`t2`

  where ((`test`.`t1`.`b1` = `test`.`t2`.`b2`) and (`test`.`t1`.`a1` = 10))

)

从查询执行计划看，子查询存在，MySQL没有进一步做子查询的优化工作。

示例二，MySQL不支持对NOT EXISTS类型的子查询的优化：

NOT EXISTS类型的相关子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE NOT EXISTS (SELECT 1 FROM t2 WHERE t1.a1= t2.a2 AND t2.a2>10);

+----+--------------------+-------+------+------+-------------+

| id | select_type    | table | type | key | Extra    |

+----+--------------------+-------+------+------+-------------+

| 1 | PRIMARY      | t1  | ALL | NULL | Using where |

| 2 | DEPENDENT SUBQUERY | t2  | ALL | NULL | Using where |

+----+--------------------+-------+------+------+-------------+

2 rows in set, 2 warnings (0.00 sec)

被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where (not(exists(

  /* select#2 */ select 1

  from `test`.`t2`

  where ((`test`.`t1`.`a1` = `test`.`t2`.`a2`) and (`test`.`t2`.`a2` > 10))))

)

从查询执行计划看，子查询存在，MySQL没有进一步做子查询的优化工作。

另外的一个NOT EXISTS类型的相关子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE NOT EXISTS (SELECT 1 FROM t2 WHERE t1.b1= t2.b2 AND t1.a1=10);

+----+--------------------+-------+------+------+-------------+

| id | select_type    | table | type | key | Extra    |

+----+--------------------+-------+------+------+-------------+

| 1 | PRIMARY      | t1  | ALL | NULL | Using where |

| 2 | DEPENDENT SUBQUERY | t2  | ALL | NULL | Using where |

+----+--------------------+-------+------+------+-------------+

2 rows in set, 3 warnings (0.00 sec)

被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where (not(exists(

  /* select#2 */ select 1

  from `test`.`t2`

  where ((`test`.`t1`.`b1` = `test`.`t2`.`b2`) and (`test`.`t1`.`a1` = 10))))

)

从查询执行计划看，子查询存在，MySQL没有进一步做子查询的优化工作。

示例三，MySQL支持对IN类型的子查询的优化，按也有不支持的情况存在：

IN非相关子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 IN (SELECT a2 FROM t2 WHERE t2.a2>10);

+----+--------------+-------------+------+------+----------------------------------------------------+

| id | select_type | table    | type | key | Extra  |

+----+--------------+-------------+------+------+----------------------------------------------------+

| 1 | SIMPLE    |  | ALL | NULL | NULL  |

| 1 | SIMPLE    | t1     | ALL | NULL | Using where; Using join buffer (Block Nested Loop) |

| 2 | MATERIALIZED | t2     | ALL | NULL | Using where  |

+----+--------------+-------------+------+------+----------------------------------------------------+

3 rows in set, 1 warning (0.00 sec)

被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1` semi join (`test`.`t2`)

where ((`test`.`t1`.`a1` = ``.`a2`) and (`test`.`t2`.`a2` > 10))

从查询执行计划看，表t2被物化后，与表t1执行了半连接（semi join）。尽管有“subquery2”这样的内容看起来是子查询，但是表t2已经被上拉到表t1层执行了半连接，所以MySQL支持IN子查询优化为半连接操作。

另外一个IN非相关子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 IN (SELECT a2 FROM t2 WHERE t2.a2=10);

+----+--------------+-------------+------+------+----------------------------------------------------+

| id | select_type | table    | type | key | Extra  |

+----+--------------+-------------+------+------+----------------------------------------------------+

| 1 | SIMPLE    |  | ALL | NULL | Using where  |

| 1 | SIMPLE    | t1     | ALL | NULL | Using where; Using join buffer (Block Nested Loop) |

| 2 | MATERIALIZED | t2     | ALL | NULL | Using where  |

+----+--------------+-------------+------+------+----------------------------------------------------+

3 rows in set, 1 warning (0.02 sec)

被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1` semi join (`test`.`t2`)

where ((``.`a2` = 10) and (`test`.`t1`.`a1` = 10) and (`test`.`t2`.`a2` = 10))

从查询执行计划看，子查询不存在，表t1和t2直接做了块嵌套循环半连接（Block Nested Loop），把子查询上拉到父查询中用嵌套循环半连接完成IN操作。另外，由于子查询上拉，使得增加连接条件“a1=a2”，而原先的条件“a2=10”可以利用常量传递优化技术，使得“a1=a2=10”，所以查询执行计划中，两个索引扫描的条件分别为：a1 = 10、a2 = 10。

另外一个IN非相关子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 IN (SELECT a2 FROM t2 WHERE t1.a1=10);

+----+-------------+-------+------+------------------------------------------------------------------+

| id | select_type | table | type | Extra      |

+----+-------------+-------+------+------------------------------------------------------------------+

| 1 | SIMPLE   | t2  | ALL | Using where; Start temporary      |

| 1 | SIMPLE   | t1  | ALL | Using where; End temporary; Using join buffer (Block Nested Loop)|

+----+-------------+-------+------+------------------------------------------------------------------+

2 rows in set, 2 warnings (0.00 sec)

被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1` semi join (`test`.`t2`)

where ((`test`.`t2`.`a2` = 10) and (`test`.`t1`.`a1` = 10))

从查询执行计划看，子子查询不存在，表t1和t2直接做了块嵌套循环连接（Block Nested Loop），但属于半连接操作（semi join），把子查询上拉到父查询中用嵌套循环半连接完成IN操作。

示例四，MySQL支持对NOT IN类型的子查询的优化

NOT IN非相关子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 NOT IN (SELECT a2 FROM t2 WHERE t2.a2>10);

+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.02 sec)

被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

`test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where (not((

  `test`.`t1`.`a1`,`test`.`t1`.`a1` in (

     (/* select#2 */

      select `test`.`t2`.`a2`

      from `test`.`t2`

      where (`test`.`t2`.`a2` > 10)

      having 1

    ),

    (

      `test`.`t1`.`a1` in  on 

      where ((`test`.`t1`.`a1` = `materialized-subquery`.`a2`))

    )

   )

  ))

)

从查询执行计划看，表t2做了子查询（SUBQUERY）。而子查询被物化（materialize）。所以，MySQL对于NOT IN子查询采用了物化的优化方式，但不支持子查询的消除。

另外一个NOT IN非相关子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 NOT IN (SELECT a2 FROM t2 WHERE t2.a2=10);

+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)

被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,`test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where (not((

  `test`.`t1`.`a1`,`test`.`t1`.`a1` in (

     (/* select#2 */

      select `test`.`t2`.`a2`

      from `test`.`t2`

      where (`test`.`t2`.`a2` = 10)

      having 1

    ),

    (

      `test`.`t1`.`a1` in  on 

      where ((`test`.`t1`.`a1` = `materialized-subquery`.`a2`))

    )

  )

  ))

)

示例五，MySQL支持对ALL类型的子查询的优化：

不相关的ALL子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 >ALL (SELECT a2 FROM t2 WHERE t2.a2>10);

+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)

被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,`test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where ((`test`.`t1`.`a1` <= (

  /* select#2 */

  select `test`.`t2`.`a2`

  from `test`.`t2`

  where (`test`.`t2`.`a2` > 10)

  )

))

从查询执行计划看，出现了子查询（SUBQUERY），但是，子查询被“<= ”操作符限制，而子查询中的被查询列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“>ALL”式的子查询优化，子查询只被执行一次即可求得最大值。

不相关的ALL子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 =ALL (SELECT a2 FROM t2 WHERE t2.a2=10);

+----+--------------------+-------+------+------+-------------+

| id | select_type    | table | type | key | Extra    |

+----+--------------------+-------+------+------+-------------+

| 1 | PRIMARY      | t1  | ALL | NULL | Using where |

| 2 | DEPENDENT SUBQUERY | t2  | ALL | NULL | Using where |

+----+--------------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)

被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,`test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where ((

  `test`.`t1`.`a1`,(

    /* select#2 */ select 1 from `test`.`t2`

    where ((`test`.`t2`.`a2` = 10) and

      (outer_field_is_not_null,

        (((`test`.`t1`.`a1`) <> 10) or (isnull(10))),

        true

      )

    )

    having (outer_field_is_not_null, (`test`.`t2`.`a2`), true)

  )

))

从查询执行计划看，出现了子查询（SUBQUERY），但是被查询优化器处理后的语句中包含“exists”，这表明MySQL对于“=ALL”式的子查询优化用“EXISTS strategy”方式优化，所以MySQL支持“=ALL”式的子查询优化。

不相关的ALL子查询，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 


+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)



被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,`test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where ((`test`.`t1`.`a1` >= 

  (/* select#2 */

    select `test`.`t2`.`a2`

    from `test`.`t2`

    where (`test`.`t2`.`a2` = 10)

  )

))



从查询执行计划看，出现了子查询（SUBQUERY），但是，子查询被“>= ”操作符限制，而子查询中的被查询列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“
 
示例六，MySQL支持对SOME类型的子查询的优化：
使用了“>SOME”式子的子查询被优化，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 >SOME (SELECT a2 FROM t2 WHERE t2.a2>10);




+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.05 sec)



被查询优化器处理后的语句为：

 /* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

   `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where ((`test`.`t1`.`a1` > (

  /* select#2 */

  select min(`test`.`t2`.`a2`)

  from `test`.`t2`

  where (`test`.`t2`.`a2` > 10)

)))



从查询执行计划看，出现了子查询（SUBQUERY），但是，子查询被“min”函数限制，而子查询中的被查询列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“>SOME”式的子查询优化，子查询只被执行一次即可求得最大值。
 
使用了“=SOME”式子的子查询被优化，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 =SOME (SELECT a2 FROM t2 WHERE t2.a2=10);




+----+--------------+-------------+------+------+----------------------------------------------------+

| id | select_type | table    | type | key | Extra  |

+----+--------------+-------------+------+------+----------------------------------------------------+

| 1 | SIMPLE    |  | ALL | NULL | Using where  |

| 1 | SIMPLE    | t1     | ALL | NULL | Using where; Using join buffer (Block Nested Loop) |

| 2 | MATERIALIZED | t2     | ALL | NULL | Using where  |

+----+--------------+-------------+------+------+----------------------------------------------------+

3 rows in set, 1 warning (0.01 sec)



被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

`test`.`t1`.`b1` AS `b1`

from `test`.`t1` semi join (`test`.`t2`)

where ((``.`a2` = 10) and (`test`.`t1`.`a1` = 10) and (`test`.`t2`.`a2` = 10))



从查询执行计划看，没有出现了子查询，表t2被物化，与表t1进行了半连接。
 
使用了“

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 


+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)



被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where (

  (

    `test`.`t1`.`a1` < (/* select#2 */

      select max(`test`.`t2`.`a2`)

      from `test`.`t2`

      where (`test`.`t2`.`a2` = 10)

    )

  )

)



从查询执行计划看，出现了子查询（SUBQUERY），但是，子查询被“max”函数限制，而子查询中的被查询列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“
 
示例七，MySQL支持对ANY类型的子查询的优化：
使用了“>ANY”式子的子查询被优化，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 >ANY (SELECT a2 FROM t2 WHERE t2.a2>10);




+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)



被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where (

  (

    `test`.`t1`.`a1` > (/* select#2 */

      select min(`test`.`t2`.`a2`)

      from `test`.`t2`

      where (`test`.`t2`.`a2` > 10)

    )

  )

)



从查询执行计划看，出现了子查询（SUBQUERY），但是，子查询被“min”函数限制，而子查询中的被查询列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“>ANY”式的子查询优化，子查询只被执行一次即可求得最小值。
 
使用了“=ANY”式子的子查询被优化，查询执行计划如下：

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 =ANY (SELECT a2 FROM t2 WHERE t2.a2>10);




+----+--------------+-------------+------+------+----------------------------------------------------+

| id | select_type | table    | type | key | Extra  |

+----+--------------+-------------+------+------+----------------------------------------------------+

| 1 | SIMPLE    |  | ALL | NULL | NULL  |

| 1 | SIMPLE    | t1     | ALL | NULL | Using where; Using join buffer (Block Nested Loop) |

| 2 | MATERIALIZED | t2     | ALL | NULL | Using where  |

+----+--------------+-------------+------+------+----------------------------------------------------+

3 rows in set, 1 warning (0.02 sec)



被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1` semi join (`test`.`t2`)

where ((`test`.`t1`.`a1` = ``.`a2`) and (`test`.`t2`.`a2` > 10))



从查询执行计划看，没有出现了子查询，表t2被物化，与表t1进行了半连接。
 
使用了“

mysql> EXPLAIN EXTENDED SELECT * FROM t1 WHERE t1.a1 10);




+----+-------------+-------+------+------+-------------+

| id | select_type | table | type | key | Extra    |

+----+-------------+-------+------+------+-------------+

| 1 | PRIMARY   | t1  | ALL | NULL | Using where |

| 2 | SUBQUERY  | t2  | ALL | NULL | Using where |

+----+-------------+-------+------+------+-------------+

2 rows in set, 1 warning (0.00 sec)



被查询优化器处理后的语句为：

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`test`.`t1`.`a1` AS `a1`,

  `test`.`t1`.`b1` AS `b1`

from `test`.`t1`

where (

  (

    `test`.`t1`.`a1` < (/* select#2 */

      select max(`test`.`t2`.`a2`)

      from `test`.`t2`

      where (`test`.`t2`.`a2` > 10)

    )

  )

)



从查询执行计划看，出现了子查询（SUBQUERY），但是，子查询被“max”函数限制，而子查询中的被查询列a2上存在唯一索引，所以可以利用索引求最值，所以MySQL支持“

                  

	 	
                    

                        
                            
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