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 ；