LEFT JOIN 关键字

LEFT JOIN 关键字

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。

语法

SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name;

或：

SELECT column_name(s)
FROM table1
LEFT OUTER JOIN table2
ON table1.column_name=table2.column_name;

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

示例

示例使用样本数据库。

该库包含 "Websites" 表：

+----+--------------+---------------------------+-------+---------+
| 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     |
| 7  | stackoverflow | http://stackoverflow.com/ |   0 | IND     |
+----+---------------+---------------------------+-------+---------+

下面是 "access_log" 网站访问记录表的数据：

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

LEFT JOIN 实例

返回所有网站及他们的访问量（如果有的话）:

SELECT Websites.name, access_log.count, access_log.date
FROM Websites
LEFT JOIN access_log
ON Websites.id=access_log.site_id
ORDER BY access_log.count DESC;

执行以上 SQL 输出结果如下：

mysql> SELECT Websites.name, access_log.count, access_log.date
    -> FROM Websites
    -> LEFT JOIN access_log
    -> ON Websites.id=access_log.site_id
    -> ORDER BY access_log.count DESC;

+---------------+---------+------------+
| name          | count   | date       |
+---------------+---------+------------+
| Facebook      |  545    | 2016-05-16 |
| Google        |   230   | 2016-05-14 |
| 菜鸟教程       |  220    | 2016-05-15 |
| Facebook      |   205   | 2016-05-14 |
| 菜鸟教程      |  201    | 2016-05-17 |
| 菜鸟教程      |   100   | 2016-05-13 |
| Google        |   45    | 2016-05-10 |
| 微博          |   13     | 2016-05-15 |
| 淘宝          |  10     | 2016-05-14 |
| stackoverflow |  NULL   | NULL       |
+---------------+--------+-------------+

注释 ： LEFT JOIN 关键字从左表（Websites）返回所有的行，即使右表（access_log）中没有匹配。