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(author) as total from files

每个作者都投稿很多，这里有重复的记录。

select distinct author from files;

有可能两个学校的教师姓名相同，结果只统计一个，出错。

select distinct author,sid  from files

统计(作者＋学校id)的组合唯一值，结果出现正确的结果，但如何知道一共有多少人呢？

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 ；

参考：https://www.cnblogs.com/wucongyun/p/6746365.html