MySQL的子查询及相关优化学习教程

2023年 4月 25日 32.3k 0

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

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 | <subquery2> | 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` = `<subquery2>`.`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 | <subquery2> | 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 ((`<subquery2>`.`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(<in_optimizer>(

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

<materialize> (/* select#2 */

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

from `test`.`t2`

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

having 1

),

<primary_index_lookup>(

`test`.`t1`.`a1` in <temporary table> on <auto_key>

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(<in_optimizer>(

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

<materialize> (/* select#2 */

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

from `test`.`t2`

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

having 1

),

<primary_index_lookup>(

`test`.`t1`.`a1` in <temporary table> on <auto_key>

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。

 

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 <not>((`test`.`t1`.`a1` <= <max>(

/* 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“<= <max>”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 <not>(<in_optimizer>(

`test`.`t1`.`a1`,<exists>(

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

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

<if>(outer_field_is_not_null,

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

true

)

)

having <if>(outer_field_is_not_null, <is_not_null_test>(`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 <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 <not>((`test`.`t1`.`a1` >= <min>

(/* 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“>= <min>”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小值。

 

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 <nop>((`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 | <subquery2> | 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 ((`<subquery2>`.`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“<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 (0.00 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 <nop>(

(

`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“<SOME”式的子查询优化,子查询只被执行一次即可求得最大值。

 

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 <nop>(

(

`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 | <subquery2> | 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` = `<subquery2>`.`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“<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 <nop>(

(

`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“<ANY”式的子查询优化,子查询只被执行一次即可求得最大值。

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