如何通过每个值的关联id找到数字重复值

Question

我有一个带有数据库表的消息系统，如下所示。 UID是用户ID，sid是发送消息的商店ID。

UID   SID   Content
1     10    "blah" 
1     11     ...
2     10     ...
3     12     ...
3     12     ...
3     10     ...

我想将用户按每个商店收到的邮件数量分组。所以输出将是

UID   NumUniqueSIDs
1     1     (corresponds to uid = 1, sid = 10)
1     1     (corresponds to uid = 1, sid = 11)
2     1     (corresponds to UID = 2, sid = 10)
3     2     (corresponds to UID = 3, sid = 12)
3     1     (corresponds to UID = 3, sid = 10)

我无法提出完成此操作的查询。有谁知道如何做到这一点？

Answer 1

以下查询会生成正确的结果：

SELECT uid, count(sid) as NumUniqueSIDs from Messages group by uid, sid