SQL RIGHT JOIN 关键字


SQL RIGHT JOIN 关键字

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。

SQL RIGHT JOIN 语法

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

或:

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

演示数据库

在本教程中,我们将使用 shulanxt 样本数据库。

下面是选自 “Websites” 表的数据:

+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | Google       | https://www.google.cm/    | 1     | USA     |
| 2  | 淘宝          | https://www.taobao.com/   | 13    | CN      |
| 3  | 树懒学堂      | http://www.shulanxt.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 |
+-----+---------+-------+------------+
9 rows in set (0.00 sec)

SQL RIGHT JOIN 实例

下面的 SQL 语句将返回网站的访问记录。

以下实例中我们把 access_log 作为左表,Websites 作为右表:

实例:

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

输出结果:

+--------------+-------+------------+
| name         | count | date       |
+--------------+-------+------------+
| Facebook     |   545 | 2016-05-16 |
| Google       |   230 | 2016-05-14 |
| 树懒学堂      |   220 | 2016-05-15 |
| Facebook     |   205 | 2016-05-14 |
| 树懒学堂      |   201 | 2016-05-17 |
| NULL         |   111 | 2016-03-09 |
| 树懒学堂      |   100 | 2016-05-13 |
| Google       |    45 | 2016-05-10 |
| 微博          |    13 | 2016-05-15 |
| 淘宝          |    10 | 2016-05-14 |
+---------------+-------+------------+

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