Mysql Join语法解析与性能分析

一．Join语法概述

join 用于多表中字段之间的联系，语法如下：

... FROM table1 INNER|LEFT|RIGHT JOIN table2 ON conditiona

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)

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)

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。

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

三.Left join

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)

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：

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)

同理，还可以模拟inner join. 如下：

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)

求差集：

根据上面的例子可以求差集，如下：

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 |+------+-----------+------+-------------+

四.Right join

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)

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。

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

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：

... FROM table1 INNER JOIN table2... FROM table1 CROSS JOIN table2... FROM table1 JOIN table2

六.Full join

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)

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。

select * from A  inner join B on B.name = A.name   left join C on C.name = B.name   left join D on D.id = C.id   where C.status>1 and D.status=1;

select * from A  inner join B on B.name = A.name   left join C on C.name = B.name and C.status>1  left join D on D.id = C.id and D.status=1//性能优秀

insert into t1(a1) select b1 from t2 where not exists(select 1 from t1 where t1.id = t2.r_id); 

insert into t1(a1)   select b1 from t2  left join (select distinct t1.id from t1 ) t1 on t1.id = t2.r_id   where t1.id is null; //性能优秀