in和inner join在大多数情况下都是返回两表的交集，但是两者还是有区别的，如下例子
mysql> select * from a;+------+------+| id | name |+------+------+| 1 | a || 2 | b || 3 | c |+------+------+
MySQL> select * from b;
+------+------+
| id | name |
+------+------+
| 1 | d |
| 1 | g |
| 2 | e |
| 4 | f |
+------+------+
mysql> select a.id, a.name from a where a.id in (select b.id from b);
+------+------+
| id | name |
+------+------+
| 1 | a |
| 2 | b |
+------+------+
mysql> select a.id, a.name from a inner join b on (a.id = b.id);
+------+------+
| id | name |
+------+------+
| 1 | a |
| 1 | a |
| 2 | b |
+------+------+
mysql> select * from a inner join b on (a.id = b.id);
+------+------+------+------+
| id | name | id | name |
+------+------+------+------+
| 1 | a | 1 | d |
| 1 | a | 1 | g |
| 2 | b | 2 | e |
+------+------+------+------+
从查询结果中可以看出，in的结果是不会有重复的，对非主键进行join时，join的结果是有重复的。如果说还有另一个区别的话就是join会产生一个两表合并的临时表，in不会产生两表合并的临时表。