为什么in嵌套循环查询结果不同？

[lumengabc]

为什么in嵌套循环查询结果不同？

mysql> select count(*) from goods
    -> where goodsid
    -> in(SELECT goodsid  FROM goodsatt WHERE cid=200);
+----------+
| count(*) |
+----------+
|   169039 |
+----------+
1 row in set (7.00 sec)

mysql> select count(*) from goods a,goodsatt b
    -> where (a.goodsid=b.goodsid and b.cid=200);
+----------+
| count(*) |
+----------+
|   172083 |
+----------+
1 row in set (2.63 sec)

[WWWWA]

A表中应该有重复的记录，检查一下