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sql 中 inner join、left join 和 right join的区别

作者:互联网

最近项目中要写个SQL,查询A表和B表都存在的情况下,很显然,用 inner join。

半天没写出来,请别人帮忙之后,问了个很菜鸟的问题,为啥要用inner join 哈哈

所以在这简单总结下:

有如下数据

mysql> select a.stuid,a.stuname,b.stuname from testa a,testb b where a.stuid=b.stuid;
+-------+---------+---------+
| stuid | stuname | stuname |
+-------+---------+---------+
|     1 | yoyo    | koko    |
|     2 | sasa    | tom     |
|     3 | tom     | jerry   |
|     4 | jerry   | yoyo    |
+-------+---------+---------+
4 rows in set (0.00 sec)

①inner join:内连接

  两个表都存在的记录(红色部分)。

  

mysql> select * from testa a inner join testb b on a.stuname = b.stuname;
+-------+---------+-------+---------+
| stuid | stuname | stuid | stuname |
+-------+---------+-------+---------+
|     3 | tom     |     2 | tom     |
|     4 | jerry   |     3 | jerry   |
|     1 | yoyo    |     4 | yoyo    |
+-------+---------+-------+---------+
3 rows in set (0.00 sec)

②left join:左连接

  取得左表全部记录 和 右表匹配的记录,如果没有匹配的显示为null(红色部分)。

  

mysql> select * from testa a left join testb b on a.stuname = b.stuname;
+-------+---------+-------+---------+
| stuid | stuname | stuid | stuname |
+-------+---------+-------+---------+
|     1 | yoyo    |     4 | yoyo    |
|     2 | sasa    |  NULL | NULL    |
|     3 | tom     |     2 | tom     |
|     4 | jerry   |     3 | jerry   |
+-------+---------+-------+---------+
4 rows in set (0.00 sec)

③right join:右连接

  取得右边全部记录 和 左表匹配的记录,如果没有匹配的显示为null(红色部分)。

  

mysql> select * from testa a right join testb b on a.stuname = b.stuname;
+-------+---------+-------+---------+
| stuid | stuname | stuid | stuname |
+-------+---------+-------+---------+
|  NULL | NULL    |     1 | koko    |
|     3 | tom     |     2 | tom     |
|     4 | jerry   |     3 | jerry   |
|     1 | yoyo    |     4 | yoyo    |
+-------+---------+-------+---------+
4 rows in set (0.00 sec)

以上。

标签:stuname,right,join,jerry,yoyo,inner,tom,stuid
来源: https://www.cnblogs.com/jianyingjie/p/11709178.html