1、使用distinct去重(適合查詢整張表的總數(shù))有多個(gè)學(xué)校＋教師投稿，需要統(tǒng)計(jì)出作者的總數(shù)select count(author) as total from files每個(gè)作者都投稿很多，這里有重復(fù)的記錄。select distinct author from files;有可能兩個(gè)學(xué)校的教師姓名相同，結(jié)果只統(tǒng)計(jì)一個(gè)，出錯(cuò)。select distinct author,sid from files統(tǒng)計(jì)(作者＋學(xué)校id)的組合唯一值，結(jié)果出現(xiàn)正確的結(jié)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 分組去重(適合根據(jù)條件分組后查詢每組的總數(shù))

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

3、記錄兩張表的數(shù)目的和，這兩個(gè)表分開(kāi)查詢

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 ；

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