MySQL的子查询及相关优化学习教程
分类:计算机教程

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)

MySQL的子查询及相关优化学习教程,mysql教程

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))

)

美洲杯赔率,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。

http://www.bkjia.com/Mysql/1072171.htmlwww.bkjia.comtruehttp://www.bkjia.com/Mysql/1072171.htmlTechArticleMySQL的子查询及相关优化学习教程,mysql教程 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...

#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)

本文由美洲杯赔率发布于计算机教程,转载请注明出处:MySQL的子查询及相关优化学习教程

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