求句SQL,多个分类下查询所属的产品

[codeyell-com]

一个产品可以属于多个分类，表结构如下：
goods_id category_id
1 3
1 4
2 3

上面的产品2属于分类3，但产品1，同时属于分类3和4.
如何能查出同时属于多个分类的产品？比如查同时属于分类3和4的产品时，结果只有1.

多谢

[yjl49]

select goods_id,count(*)
from (
select goods_id,category_id
from mytable
group by goods_id,category_id
) r
group by goods_id
having count(*)>1;

[ACMAIN_CHM]

select a.* 
from 表结构如下 a inner join 表结构如下 b on a.goods_id=b.goods_id
where a.category_id=3 and b.category_id=4

[ACMAIN_CHM]

如果条件比较多的话，则可以如下。

select a.* 
from 表结构如下 a 
where category_id=3 
and exists (select 1 from 表结构如下 where goods_id=a.goods_id and category_id=4)
and exists (select 1 from 表结构如下 where goods_id=a.goods_id and category_id=5)
...
and exists (select 1 from 表结构如下 where goods_id=a.goods_id and category_id=X)

[codeyell-com]

多谢回复。

select 1 from ...
这个1是什么意思？