[笔记][SQL] 连接join

在学习菜鸟教程里的mysql教程时，对左右连接的结果有点不解。

其中有如下两个表：

runoob_tbl:
+-----------+--------------+---------------+-----------------+
| runoob_id | runoob_title | runoob_author | submission_date |
+-----------+--------------+---------------+-----------------+
|         1 | 学习php      | 菜鸟教程      | 2017-04-12      |
|         2 | 学习mysql    | 菜鸟教程      | 2017-04-12      |
|         3 | 学习java     | runoob.com    | 2015-05-01      |
|         4 | 学习python   | runoob.com    | 2016-03-06      |
|         5 | 学习c        | fk            | 2017-03-06      |
+-----------+--------------+---------------+-----------------+

tcount_tbl:
+---------------+--------------+
| runoob_author | runoob_count |
+---------------+--------------+
| 菜鸟教程      |           10 |
| runoob.com    |           20 |
| google        |           22 |
+---------------+--------------+

执行右连接语句后：

select a.runoob_id, a.runoob_author, b.runoob_count 
from runoob_tbl a right join tcount_tbl b 
on a.runoob_author = b.runoob_author;

得到的结果是：

+-----------+---------------+--------------+
| runoob_id | runoob_author | runoob_count |
+-----------+---------------+--------------+
|         1 | 菜鸟教程      |           10 |
|         2 | 菜鸟教程      |           10 |
|         3 | runoob.com    |           20 |
|         4 | runoob.com    |           20 |
|      null | null          |           22 |
+-----------+---------------+--------------+

我对这个结果感到很奇怪，右连接是会返回右表的所有行，不管有无匹配，但右表runoob_author明明有google这个数据，为什么会是null。

试着把所有列打印出来：

+-----------+--------------+---------------+-----------------+---------------+--------------+
| runoob_id | runoob_title | runoob_author | submission_date | runoob_author | runoob_count |
+-----------+--------------+---------------+-----------------+---------------+--------------+
|         1 | 学习php      | 菜鸟教程      | 2017-04-12      | 菜鸟教程      |           10 |
|         2 | 学习mysql    | 菜鸟教程      | 2017-04-12      | 菜鸟教程      |           10 |
|         3 | 学习java     | runoob.com    | 2015-05-01      | runoob.com    |           20 |
|         4 | 学习python   | runoob.com    | 2016-03-06      | runoob.com    |           20 |
|      null | null         | null          | null            | google        |           22 |
+-----------+--------------+---------------+-----------------+---------------+--------------+

可以看到google还是在的。这才明白原来select的是左表的runoob_author，所以打印出来才会是null。只要换成b.runoob_author就可以看到期望的结果了：

+---------------+--------------+
| runoob_author | runoob_count |
+---------------+--------------+
| 菜鸟教程      |           10 |
| runoob.com    |           20 |
| google        |           22 |
+---------------+--------------+