MySQL多表关联查询

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SQL 连接（JOIN）子句用于把来自两个或多个表的行结合起来，基于这些表之间的共同字段。连接的结果可以在逻辑上看作是由SELECT语句指定的列组成的新表。左连接与右连接的左右指的是以两张表中的哪一张为基准，它们都是外连接。外连接就好像是为非基准表添加了一行全为空值的万能行，用来与基准表中找不到匹配的行进行匹配。假设两个没有空值的表进行左连接，左表是基准表，左表的所有行都出现在结果中，右表则可能因为无法与基准表匹配而出现是空值的字段。

不同的 SQL JOIN

在我们继续讲解实例之前，我们先列出您可以使用的不同的 SQL JOIN 类型：

INNER JOIN：如果表中有至少一个匹配，则返回行

LEFT JOIN：即使右表中没有匹配，也从左表返回所有的行
RIGHT JOIN：即使左表中没有匹配，也从右表返回所有的行
FULL JOIN：只要其中一个表中存在匹配，则返回行（MySQL不支持FULL JOIN）

实例表1：

mysql> select * from websites;
+----+---------------+---------------------------+-------+---------+
| id | name          | url                       | alexa | country |
+----+---------------+---------------------------+-------+---------+
|  1 | Google        | https://www.google.cm/    |     1 | USA     |
|  2 | 淘宝          | https://www.taobao.com/   |    13 | CN      |
|  3 | 菜鸟教程      | http://www.runoob.com/    |  4689 | CN      |
|  4 | 微博          | http://weibo.com/         |    20 | CN      |
|  5 | Facebook      | https://www.facebook.com/ |     3 | USA     |
|  6 | stackoverflow | http://stackoverflow.com/ |     0 | IND     |
|  7 | 百度          | https://www.baidu.com/    |     4 | CN      |
+----+---------------+---------------------------+-------+---------+
7 rows in set

实例表2:

mysql> select * from access_log;
+-----+---------+-------+------------+
| aid | site_id | count | date       |
+-----+---------+-------+------------+
|   1 |       1 |    45 | 2016-05-10 |
|   2 |       3 |   100 | 2016-05-13 |
|   3 |       1 |   230 | 2016-05-14 |
|   4 |       2 |    10 | 2016-05-14 |
|   5 |       5 |   205 | 2016-05-14 |
|   6 |       4 |    13 | 2016-05-15 |
|   7 |       3 |   220 | 2016-05-15 |
|   8 |       5 |   545 | 2016-05-16 |
|   9 |       3 |   201 | 2016-05-17 |
+-----+---------+-------+------------+
9 rows in set

取交集：

mysql> SELECT w.name, a.count, a.date FROM websites w, access_log a WHERE w.id = a.site_id;
+----------+-------+------------+
| name     | count | date       |
+----------+-------+------------+
| Google   |    45 | 2016-05-10 |
| 菜鸟教程 |   100 | 2016-05-13 |
| Google   |   230 | 2016-05-14 |
| 淘宝     |    10 | 2016-05-14 |
| Facebook |   205 | 2016-05-14 |
| 微博     |    13 | 2016-05-15 |
| 菜鸟教程 |   220 | 2016-05-15 |
| Facebook |   545 | 2016-05-16 |
| 菜鸟教程 |   201 | 2016-05-17 |
+----------+-------+------------+
9 rows in set

mysql> SELECT w.name, a.count, a.date FROM websites w INNER JOIN access_log a ON w.id = a.site_id;
+----------+-------+------------+
| name     | count | date       |
+----------+-------+------------+
| Google   |    45 | 2016-05-10 |
| 菜鸟教程 |   100 | 2016-05-13 |
| Google   |   230 | 2016-05-14 |
| 淘宝     |    10 | 2016-05-14 |
| Facebook |   205 | 2016-05-14 |
| 微博     |    13 | 2016-05-15 |
| 菜鸟教程 |   220 | 2016-05-15 |
| Facebook |   545 | 2016-05-16 |
| 菜鸟教程 |   201 | 2016-05-17 |
+----------+-------+------------+
9 rows in set

注释：
INNER JOIN 与 JOIN 是相同的。

取左并集：

注释：在某些数据库中，LEFT JOIN 称为 LEFT OUTER JOIN。

LEFT JOIN 关键字从左表（table1）返回所有的行，即使右表（table2）中没有匹配。如果右表中没有匹配，则结果为 NULL

mysql> SELECT w.name, a.count, a.date FROM websites w LEFT JOIN access_log a ON w.id = a.site_id;
+---------------+-------+------------+
| name          | count | date       |
+---------------+-------+------------+
| Google        |    45 | 2016-05-10 |
| 菜鸟教程      |   100 | 2016-05-13 |
| Google        |   230 | 2016-05-14 |
| 淘宝          |    10 | 2016-05-14 |
| Facebook      |   205 | 2016-05-14 |
| 微博          |    13 | 2016-05-15 |
| 菜鸟教程      |   220 | 2016-05-15 |
| Facebook      |   545 | 2016-05-16 |
| 菜鸟教程      |   201 | 2016-05-17 |
| stackoverflow | NULL  | NULL       |
| 百度          | NULL  | NULL       |
+---------------+-------+------------+
11 rows in set

取右并集：

注释：在某些数据库中，RIGHT JOIN 称为 RIGHT OUTER JOIN。

RIGHT JOIN 关键字从右表（table2）返回所有的行，即使左表（table1）中没有匹配。如果左表中没有匹配，则结果为 NULL。

mysql> SELECT Websites.name, access_log.count, access_log.date
FROM access_log
RIGHT JOIN Websites
ON access_log.site_id=Websites.id;
+---------------+-------+------------+
| name          | count | date       |
+---------------+-------+------------+
| Google        |    45 | 2016-05-10 |
| 菜鸟教程      |   100 | 2016-05-13 |
| Google        |   230 | 2016-05-14 |
| 淘宝          |    10 | 2016-05-14 |
| Facebook      |   205 | 2016-05-14 |
| 微博          |    13 | 2016-05-15 |
| 菜鸟教程      |   220 | 2016-05-15 |
| Facebook      |   545 | 2016-05-16 |
| 菜鸟教程      |   201 | 2016-05-17 |
| stackoverflow | NULL  | NULL       |
| 百度          | NULL  | NULL       |
+---------------+-------+------------+
11 rows in set

取全集：

MySQL Full Join的实现因为MySQL不支持FULL JOIN,下面是替代方法

left join + union(可去除重复数据)+ right join

select * from A left join B on A.id = B.id (where 条件）union

select * from A right join B on A.id = B.id （where条件);

mysql> SELECT * FROM websites LEFT JOIN access_log ON Websites.id=access_log.site_id 
UNION SELECT * FROM websites RIGHT JOIN access_log ON Websites.id=access_log.site_id;
+----+---------------+---------------------------+-------+---------+------+---------+-------+------------+
| id | name          | url                       | alexa | country | aid  | site_id | count | date       |
+----+---------------+---------------------------+-------+---------+------+---------+-------+------------+
|  1 | Google        | https://www.google.cm/    |     1 | USA     |    1 |       1 |    45 | 2016-05-10 |
|  3 | 菜鸟教程      | http://www.runoob.com/    |  4689 | CN      |    2 |       3 |   100 | 2016-05-13 |
|  1 | Google        | https://www.google.cm/    |     1 | USA     |    3 |       1 |   230 | 2016-05-14 |
|  2 | 淘宝          | https://www.taobao.com/   |    13 | CN      |    4 |       2 |    10 | 2016-05-14 |
|  5 | Facebook      | https://www.facebook.com/ |     3 | USA     |    5 |       5 |   205 | 2016-05-14 |
|  4 | 微博          | http://weibo.com/         |    20 | CN      |    6 |       4 |    13 | 2016-05-15 |
|  3 | 菜鸟教程      | http://www.runoob.com/    |  4689 | CN      |    7 |       3 |   220 | 2016-05-15 |
|  5 | Facebook      | https://www.facebook.com/ |     3 | USA     |    8 |       5 |   545 | 2016-05-16 |
|  3 | 菜鸟教程      | http://www.runoob.com/    |  4689 | CN      |    9 |       3 |   201 | 2016-05-17 |
|  6 | stackoverflow | http://stackoverflow.com/ |     0 | IND     | NULL | NULL    | NULL  | NULL       |
|  7 | 百度          | https://www.baidu.com/    |     4 | CN      | NULL | NULL    | NULL  | NULL       |
+----+---------------+---------------------------+-------+---------+------+---------+-------+------------+
11 rows in set

SQL UNION 操作符合并两个或多个 SELECT 语句的结果。

SQL UNION 操作符

UNION 操作符用于合并两个或多个 SELECT 语句的结果集。

请注意，UNION 内部的每个 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每个 SELECT 语句中的列的顺序必须相同。

SQL UNION 语法

     SELECT 

    column_name(s) FROM 

    table1

 UNION

 SELECT 

    column_name(s) FROM 

    table2;

注释：默认地，UNION 操作符选取不同的值。如果允许重复的值，请使用 UNION ALL。

SQL UNION ALL 语法

     SELECT 

    column_name(s) FROM 

    table1

 UNION ALL

 SELECT 

    column_name(s) FROM 

    table2;

注释：UNION 结果集中的列名总是等于 UNION 中第一个 SELECT 语句中的列名。

mysql> SELECT * FROM websites LEFT JOIN access_log ON Websites.id=access_log.site_id 
UNION ALL SELECT * FROM websites RIGHT JOIN access_log ON Websites.id=access_log.site_id;
+----+---------------+---------------------------+-------+---------+------+---------+-------+------------+
| id | name          | url                       | alexa | country | aid  | site_id | count | date       |
+----+---------------+---------------------------+-------+---------+------+---------+-------+------------+
|  1 | Google        | https://www.google.cm/    |     1 | USA     |    1 |       1 |    45 | 2016-05-10 |
|  3 | 菜鸟教程      | http://www.runoob.com/    |  4689 | CN      |    2 |       3 |   100 | 2016-05-13 |
|  1 | Google        | https://www.google.cm/    |     1 | USA     |    3 |       1 |   230 | 2016-05-14 |
|  2 | 淘宝          | https://www.taobao.com/   |    13 | CN      |    4 |       2 |    10 | 2016-05-14 |
|  5 | Facebook      | https://www.facebook.com/ |     3 | USA     |    5 |       5 |   205 | 2016-05-14 |
|  4 | 微博          | http://weibo.com/         |    20 | CN      |    6 |       4 |    13 | 2016-05-15 |
|  3 | 菜鸟教程      | http://www.runoob.com/    |  4689 | CN      |    7 |       3 |   220 | 2016-05-15 |
|  5 | Facebook      | https://www.facebook.com/ |     3 | USA     |    8 |       5 |   545 | 2016-05-16 |
|  3 | 菜鸟教程      | http://www.runoob.com/    |  4689 | CN      |    9 |       3 |   201 | 2016-05-17 |
|  6 | stackoverflow | http://stackoverflow.com/ |     0 | IND     | NULL | NULL    | NULL  | NULL       |
|  7 | 百度          | https://www.baidu.com/    |     4 | CN      | NULL | NULL    | NULL  | NULL       |
|  1 | Google        | https://www.google.cm/    |     1 | USA     |    1 |       1 |    45 | 2016-05-10 |
|  3 | 菜鸟教程      | http://www.runoob.com/    |  4689 | CN      |    2 |       3 |   100 | 2016-05-13 |
|  1 | Google        | https://www.google.cm/    |     1 | USA     |    3 |       1 |   230 | 2016-05-14 |
|  2 | 淘宝          | https://www.taobao.com/   |    13 | CN      |    4 |       2 |    10 | 2016-05-14 |
|  5 | Facebook      | https://www.facebook.com/ |     3 | USA     |    5 |       5 |   205 | 2016-05-14 |
|  4 | 微博          | http://weibo.com/         |    20 | CN      |    6 |       4 |    13 | 2016-05-15 |
|  3 | 菜鸟教程      | http://www.runoob.com/    |  4689 | CN      |    7 |       3 |   220 | 2016-05-15 |
|  5 | Facebook      | https://www.facebook.com/ |     3 | USA     |    8 |       5 |   545 | 2016-05-16 |
|  3 | 菜鸟教程      | http://www.runoob.com/    |  4689 | CN      |    9 |       3 |   201 | 2016-05-17 |
+----+---------------+---------------------------+-------+---------+------+---------+-------+------------+
20 rows in set