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Mysql学习MySQL的子查询及相关优化学习教程

发布时间：2020-12-12 01:24:11 所属栏目：MySql教程 来源：网络整理
导读：《Mysql学习MySQL的子查询及相关优化学习教程》要点： 本文介绍了Mysql学习MySQL的子查询及相关优化学习教程，希望对您有用。如果有疑问，可以联系我们。 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学习 #不用order by 来查

《Mysql学习MySQL的子查询及相关优化学习教程》要点：
本文介绍了Mysql学习MySQL的子查询及相关优化学习教程，希望对您有用。如果有疑问，可以联系我们。

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学习

#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：
MYSQL学习

#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)
MYSQL学习

#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.MYSQL学习

?MYSQL学习

示例一,MySQL不支持对EXISTS类型的子查询的优化：MYSQL学习

EXISTS类型的相关子查询,查询执行计划如下：MYSQL学习

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：MYSQL学习

/* 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.MYSQL学习

另外的一个EXISTS类型的相关子查询,查询执行计划如下：MYSQL学习

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：MYSQL学习

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`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,MySQL没有进一步做子查询的优化工作.MYSQL学习

?MYSQL学习

示例二,MySQL不支持对NOT EXISTS类型的子查询的优化：MYSQL学习

NOT EXISTS类型的相关子查询,查询执行计划如下：MYSQL学习

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,`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,MySQL没有进一步做子查询的优化工作.MYSQL学习

?MYSQL学习

另外的一个NOT EXISTS类型的相关子查询,查询执行计划如下：MYSQL学习

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：MYSQL学习

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,`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,MySQL没有进一步做子查询的优化工作.MYSQL学习

?MYSQL学习

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：MYSQL学习

IN非相关子查询,查询执行计划如下：MYSQL学习

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：MYSQL学习

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

?MYSQL学习

另外一个IN非相关子查询,查询执行计划如下：MYSQL学习

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：MYSQL学习

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

?MYSQL学习

另外一个IN非相关子查询,查询执行计划如下：MYSQL学习

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`.`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.MYSQL学习

?MYSQL学习

示例四,MySQL支持对NOT IN类型的子查询的优化MYSQL学习

NOT IN非相关子查询,查询执行计划如下：MYSQL学习

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,`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.MYSQL学习

?MYSQL学习

另外一个NOT IN非相关子查询,查询执行计划如下：MYSQL学习

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,`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,但不支持子查询的消除.MYSQL学习

?MYSQL学习

示例五,MySQL支持对ALL类型的子查询的优化：MYSQL学习

不相关的ALL子查询,查询执行计划如下：MYSQL学习

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,`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.MYSQL学习

?MYSQL学习

不相关的ALL子查询,查询执行计划如下：MYSQL学习

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,`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,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.MYSQL学习

?MYSQL学习

不相关的ALL子查询,查询执行计划如下：MYSQL学习

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,`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,子查询被“>= <min>”操作符限制,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.MYSQL学习

?MYSQL学习

示例六,MySQL支持对SOME类型的子查询的优化：MYSQL学习

使用了“>SOME”式子的子查询被优化,查询执行计划如下：MYSQL学习

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：MYSQL学习

 /* select#1 */ select `test`.`t1`.`id1` AS `id1`,`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,子查询被“min”函数限制,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.MYSQL学习

?MYSQL学习

使用了“=SOME”式子的子查询被优化,查询执行计划如下：MYSQL学习

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：MYSQL学习

/* select#1 */ select `test`.`t1`.`id1` AS `id1`,没有出现了子查询,表t2被物化,与表t1进行了半连接.MYSQL学习


?MYSQL学习


使用了“<SOME”式子的子查询被优化,查询执行计划如下：MYSQL学习



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,`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,子查询被“max”函数限制,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.MYSQL学习


?MYSQL学习


示例七,MySQL支持对ANY类型的子查询的优化：MYSQL学习


使用了“>ANY”式子的子查询被优化,查询执行计划如下：MYSQL学习



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,`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,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.MYSQL学习


?MYSQL学习


使用了“=ANY”式子的子查询被优化,查询执行计划如下：MYSQL学习



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,与表t1进行了半连接.MYSQL学习


?MYSQL学习


使用了“<ANY”式子的子查询被优化,查询执行计划如下：MYSQL学习



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,`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,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.MYSQL学习
                        
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