MySQL怎么实现数据去重

select count(author) as total from files

select distinct author from files;

select distinct author,sid from files

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

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

SELECT SUM©

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 ；