mysql中left join 和right join和join

存在两张表，数据如下：

mysql> select * from a;
+----+------+
| id | name |
+----+------+
|  1 | aaa  |
|  2 | bbb  |
|  3 | ccc  |
+----+------+

mysql> select * from b;
+----+------+
| id | name |
+----+------+
|  1 | aaa  |
|  2 | bbb  |
|  4 | ccc  |
+----+------+

mysql> select * from a join b on a.id=b.id;
+----+------+----+------+
| id | name | id | name |
+----+------+----+------+
|  1 | aaa  |  1 | aaa  |
|  2 | bbb  |  2 | bbb  |
+----+------+----+------+

存在的关联关系的结果，join相当于我们平时用的where，就是把两张表中同时满足a.id=b.id的数据找出来；

mysql> select * from a left join b on a.id=b.id;
+----+------+------+------+
| id | name | id   | name |
+----+------+------+------+
|  1 | aaa  |    1 | aaa  |
|  2 | bbb  |    2 | bbb  |
|  3 | ccc  | NULL | NULL |
+----+------+------+------+

可见，left join是以左表（a）为参考对象，相当于做一个for循环，把a表的数据一条一条的读取出来，然后根据a.id=b.id的条件到b表中查找数据，在b表中找不到数据的，就填充NULL；

mysql> select * from a right join b on a.id=b.id;
+------+------+----+------+
| id   | name | id | name |
+------+------+----+------+
|    1 | aaa  |  1 | aaa  |
|    2 | bbb  |  2 | bbb  |
| NULL | NULL |  4 | ccc  |
+------+------+----+------+

可见，right join跟left join刚好相反，是以右表（b）为参考对象，相当于做一个for循环，把b表的数据一条一条的读取出来，然后根据a.id=b.id的条件到a表中查找数据，在a表中找不到数据的，就填充NULL；用right join 还不如换个顺序用left join：select * from b left join a on b.id=a.id;