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Mysql Join语法解析与性能分析

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Release: 2016-06-01 13:13:00
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一.Join语法概述

join 用于多表中字段之间的联系,语法如下:

<code>... FROM table1 INNER|LEFT|RIGHT JOIN table2 ON conditiona</code>
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table1:左表;table2:右表。

JOIN 按照功能大致分为如下三类:

INNER JOIN(内连接,或等值连接):取得两个表中存在连接匹配关系的记录。

LEFT JOIN(左连接):取得左表(table1)完全记录,即是右表(table2)并无对应匹配记录。

RIGHT JOIN(右连接):与 LEFT JOIN 相反,取得右表(table2)完全记录,即是左表(table1)并无匹配对应记录。

注意:mysql不支持Full join,不过可以通过UNION 关键字来合并 LEFT JOIN 与 RIGHT JOIN来模拟FULL join.

接下来给出一个列子用于解释下面几种分类。如下两个表(A,B)

<code>mysql> select A.id,A.name,B.name from A,B where A.id=B.id;+----+-----------+-------------+| id | name | name |+----+-----------+-------------+|1 | Pirate | Rutabaga||2 | Monkey| Pirate||3 | Ninja | Darth Vader ||4 | Spaghetti| Ninja |+----+-----------+-------------+4 rows in set (0.00 sec)</code>
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二.Inner join

内连接,也叫等值连接,inner join产生同时符合A和B的一组数据。

<code>mysql> select * from A inner join B on A.name = B.name;+----+--------+----+--------+| id | name | id | name |+----+--------+----+--------+|1 | Pirate |2 | Pirate ||3 | Ninja|4 | Ninja|+----+--------+----+--------+</code>
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三.Left join

<code>mysql> select * from A left join B on A.name = B.name;#或者:select * from A left outer join B on A.name = B.name;+----+-----------+------+--------+| id | name| id | name |+----+-----------+------+--------+|1 | Pirate|2 | Pirate ||2 | Monkey| NULL | NULL ||3 | Ninja |4 | Ninja||4 | Spaghetti | NULL | NULL |+----+-----------+------+--------+4 rows in set (0.00 sec)</code>
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left join,(或left outer join:在Mysql中两者等价,推荐使用left join.)左连接从左表(A)产生一套完整的记录,与匹配的记录(右表(B)) .如果没有匹配,右侧将包含null。

如果想只从左表(A)中产生一套记录,但不包含右表(B)的记录,可以通过设置where语句来执行,如下:

<code>mysql> select * from A left join B on A.name=B.name where A.id is null or B.id is null;+----+-----------+------+------+| id | name| id | name |+----+-----------+------+------+|2 | Monkey| NULL | NULL ||4 | Spaghetti | NULL | NULL |+----+-----------+------+------+2 rows in set (0.00 sec)</code>
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同理,还可以模拟inner join. 如下:

<code>mysql> select * from A left join B on A.name=B.name where A.id is not null and B.id is not null;+----+--------+------+--------+| id | name | id | name |+----+--------+------+--------+|1 | Pirate |2 | Pirate ||3 | Ninja|4 | Ninja|+----+--------+------+--------+2 rows in set (0.00 sec)</code>
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求差集:

根据上面的例子可以求差集,如下:

<code>SELECT * FROM A LEFT JOIN B ON A.name = B.nameWHERE B.id IS NULLunionSELECT * FROM A right JOIN B ON A.name = B.nameWHERE A.id IS NULL;# 结果	+------+-----------+------+-------------+| id | name	| id | name		|+------+-----------+------+-------------+|	2 | Monkey	| NULL | NULL		||	4 | Spaghetti | NULL | NULL		|| NULL | NULL	|	1 | Rutabaga	|| NULL | NULL	|	3 | Darth Vader |+------+-----------+------+-------------+</code>
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四.Right join

<code>mysql> select * from A right join B on A.name = B.name;+------+--------+----+-------------+| id | name | id | name|+------+--------+----+-------------+| NULL | NULL |1 | Rutabaga||1 | Pirate |2 | Pirate|| NULL | NULL |3 | Darth Vader ||3 | Ninja|4 | Ninja |+------+--------+----+-------------+4 rows in set (0.00 sec)</code>
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同left join。

五.Cross join

cross join:交叉连接,得到的结果是两个表的乘积,即笛卡尔积

笛卡尔(Descartes)乘积又叫直积。假设集合A={a,b},集合B={0,1,2},则两个集合的笛卡尔积为{(a,0),(a,1),(a,2),(b,0),(b,1), (b,2)}。可以扩展到多个集合的情况。类似的例子有,如果A表示某学校学生的集合,B表示该学校所有课程的集合,则A与B的笛卡尔积表示所有可能的选课情况。

<code>mysql> select * from A cross join B;+----+-----------+----+-------------+| id | name| id | name|+----+-----------+----+-------------+|1 | Pirate|1 | Rutabaga||2 | Monkey|1 | Rutabaga||3 | Ninja |1 | Rutabaga||4 | Spaghetti |1 | Rutabaga||1 | Pirate|2 | Pirate||2 | Monkey|2 | Pirate||3 | Ninja |2 | Pirate||4 | Spaghetti |2 | Pirate||1 | Pirate|3 | Darth Vader ||2 | Monkey|3 | Darth Vader ||3 | Ninja |3 | Darth Vader ||4 | Spaghetti |3 | Darth Vader ||1 | Pirate|4 | Ninja ||2 | Monkey|4 | Ninja ||3 | Ninja |4 | Ninja ||4 | Spaghetti |4 | Ninja |+----+-----------+----+-------------+16 rows in set (0.00 sec)#再执行:mysql> select * from A inner join B; 试一试#在执行mysql> select * from A cross join B on A.name = B.name; 试一试</code>
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实际上,在 MySQL 中(仅限于 MySQL) CROSS JOIN 与 INNER JOIN 的表现是一样的,在不指定 ON 条件得到的结果都是笛卡尔积,反之取得两个表完全匹配的结果。 INNER JOIN 与 CROSS JOIN 可以省略 INNER 或 CROSS 关键字,因此下面的 SQL 效果是一样的:

<code>... FROM table1 INNER JOIN table2... FROM table1 CROSS JOIN table2... FROM table1 JOIN table2</code>
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六.Full join

<code>mysql> select * from A left join B on B.name = A.name 	-> union 	-> select * from A right join B on B.name = A.name;+------+-----------+------+-------------+| id | name	| id | name		|+------+-----------+------+-------------+|	1 | Pirate	|	2 | Pirate	||	2 | Monkey	| NULL | NULL		||	3 | Ninja	 |	4 | Ninja	 ||	4 | Spaghetti | NULL | NULL		|| NULL | NULL	|	1 | Rutabaga	|| NULL | NULL	|	3 | Darth Vader |+------+-----------+------+-------------+6 rows in set (0.00 sec)</code>
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全连接产生的所有记录(双方匹配记录)在表A和表B。如果没有匹配,则对面将包含null。

七.性能优化

1.显示(explicit) inner join VS 隐式(implicit) inner join

如:

<code>select * fromtable a inner join table bon a.id = b.id;</code>
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VS

<code>select a.*, b.*from table a, table bwhere a.id = b.id;</code>
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我在数据库中比较(10w数据)得之,它们用时几乎相同,第一个是显示的inner join,后一个是隐式的inner join。

参照:Explicit vs implicit SQL joins

2.left join/right join VS inner join

尽量用inner join.避免 LEFT JOIN 和 NULL.

在使用left join(或right join)时,应该清楚的知道以下几点:

(1). on与 where的执行顺序

ON 条件(“A LEFT JOIN B ON 条件表达式”中的ON)用来决定如何从 B 表中检索数据行。如果 B 表中没有任何一行数据匹配 ON 的条件,将会额外生成一行所有列为 NULL 的数据,在匹配阶段 WHERE 子句的条件都不会被使用。仅在匹配阶段完成以后,WHERE 子句条件才会被使用。它将从匹配阶段产生的数据中检索过滤。

所以我们要注意:在使用Left (right) join的时候,一定要在先给出尽可能多的匹配满足条件,减少Where的执行。如:

PASS

<code>select * from Ainner join B on B.name = A.nameleft join C on C.name = B.nameleft join D on D.id = C.idwhere C.status>1 and D.status=1;</code>
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Great

<code>select * from Ainner join B on B.name = A.nameleft join C on C.name = B.name and C.status>1left join D on D.id = C.id and D.status=1</code>
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从上面例子可以看出,尽可能满足ON的条件,而少用Where的条件。从执行性能来看第二个显然更加省时。

(2).注意ON 子句和 WHERE 子句的不同

如作者举了一个列子:

<code>mysql> SELECT * FROM product LEFT JOIN product_details	ON (product.id = product_details.id)	AND product_details.id=2;+----+--------+------+--------+-------+| id | amount | id | weight | exist |+----+--------+------+--------+-------+|1 |100 | NULL | NULL |NULL ||2 |200 |2 | 22 | 0 ||3 |300 | NULL | NULL |NULL ||4 |400 | NULL | NULL |NULL |+----+--------+------+--------+-------+4 rows in set (0.00 sec)mysql> SELECT * FROM product LEFT JOIN product_details	ON (product.id = product_details.id)	WHERE product_details.id=2;+----+--------+----+--------+-------+| id | amount | id | weight | exist |+----+--------+----+--------+-------+|2 |200 |2 | 22 | 0 |+----+--------+----+--------+-------+1 row in set (0.01 sec)</code>
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从上可知,第一条查询使用 ON 条件决定了从 LEFT JOIN的 product_details表中检索符合的所有数据行。第二条查询做了简单的LEFT JOIN,然后使用 WHERE 子句从 LEFT JOIN的数据中过滤掉不符合条件的数据行。

(3).尽量避免子查询,而用join

往往性能这玩意儿,更多时候体现在数据量比较大的时候,此时,我们应该避免复杂的子查询。如下:

PASS

<code>insert into t1(a1) select b1 from t2 where not exists(select 1 from t1 where t1.id = t2.r_id); </code>
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Great

<code>insert into t1(a1)select b1 from t2left join (select distinct t1.id from t1 ) t1 on t1.id = t2.r_id where t1.id is null;</code>
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这个可以参考mysql的exists与inner join 和 not exists与 left join 性能差别惊人

八.参考:

A Visual Explanation of SQL Joins

五种提高 SQL 性能的方法

关于 MySQL LEFT JOIN 你可能需要了解的三点

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