引用：

http://blog.sina.com.cn/s/blog_6c9d65a10101bkgk.html
http://www.jb51.net/article/39302.htm

1、使用distinct去重(适合查询整张表的总数)
有多个学校＋教师投稿，需要统计出作者的总数

select count(distinct author,sid) as total from files

2、group by 分组去重(适合根据条件分组后查询每组的总数)

select author, count(distinct id) from files group by sid

3、记录两张表的数目的和，这两个表分开查询

SELECT SUM(c) 
FROM
  (SELECT COUNT(DISTINCT from_user_id, message_id) c
  FROM im_message 
  WHERE dr = 0  AND message_status = 2  AND user_type = 1  AND to_user_id = 2 
  UNION ALL 
  SELECT COUNT(DISTINCT group_id, message_id) c
  FROM im_messagerefgroup 
  WHERE dr = 0  AND user_id = 2
  ) 
AS temp ；