1、使用distinct去重(適合查詢整張表的總數)有多個學校＋教師投稿，需要統計出作者的總數select count(author) as total from files每個作者都投稿很多，這里有重復的記錄。select distinct author from files;有可能兩個學校的教師姓名相同，結果只統計一個，出錯。select distinct author,sid from files統計(作者＋學校id)的組合唯一值，結果出現正確的結果，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(適合根據條件分組后查詢每組的總數)

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

3、記錄兩張表的數目的和，這兩個表分開查詢

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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