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;
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;
FROM table1
RIGHT OUTER JOIN table2
ON table1.column_name=table2.column_name;
注释:在某些数据库中,RIGHT JOIN 称为 RIGHT OUTER JOIN。
演示数据库
在本教程中,我们将使用 RUNOOB 样本数据库。
操作前先先在 access_log 表添加一条数据,该数据在 Websites 表没有对应的数据:
INSERT INTO `access_log` (`aid`, `site_id`, `count`, `date`) VALUES ('10', '6', '111', '2016-03-09');
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 | 开源教程 | / | 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 | | 10 | 6 | 111 | 2016-03-19 | +-----+---------+-------+------------+ 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 Websites 作为左表,access_log 作为右表:
实例
SELECT websites.name, access_log.count, access_log.date
FROM websites
RIGHT JOIN access_log
ON access_log.site_id=websites.id
ORDER BY access_log.count DESC;
执行以上 SQL 输出结果如下:
注释:RIGHT JOIN 关键字从右表(access_log)返回所有的行,即使左表(Websites)中没有匹配。