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。
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 | 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 | +-----+---------+-------+------------+ 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;
执行以上 SQL 输出结果如下:
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。
点我分享笔记