SQL删除重复数据，只保留一行

在sql的使用中，我们总是碰到需要删除重复数据的情况，但是又不能全部删除完，必须要保留至少一个重复的数据。重复的记录根据两个字段uid, qid判断(实际使用中可以拓展为多个)。

例如：

id	uid	qid
1	1	1
2	1	2
3	2	2
4	2	2
5	3	3
6	2	2

在上述的表中第三行和第四行重复，我们要选择一行删除，流程如下：

选择重复的行

select *,count(*) 
from A group by uid, qid 
having count(*)>1;

结果如下：

id	uid	qid	count(*)
3	2	2	3

使用in来找到我们想要的ID

SELECT *
FROM A 
WHERE (uid, qid) IN 
(SELECT A.`uid`,A.`qid` 
FROM A 
GROUP BY A.`uid`,A.`qid` 
HAVING COUNT(*)>1)

得到的结果如下：

id	uid	qid
3	2	2
4	2	2
6	2	2

选出要删除的值

SELECT * 
FROM A 
WHERE (uid, qid) IN 
(SELECT `uid`,`qid` 
FROM A 
GROUP BY A.`uid`,A.`qid` 
HAVING COUNT(*) > 1) 
AND idNOT IN 
(SELECT MIN(id) 
FROM A 
GROUP BY A.`uid`,A.`qid` 
HAVING COUNT(*) > 1) ;

结果是保留id最小的值，其他选项全部选出。

删除值

//创建中间表 
CREATE TABLE F(id INTEGER, uid INTEGER, qid INTEGER); 
//将要删除的数据插入中间表 
INSERT INTO F ( SELECT * FROM A WHERE (uid, qid) IN (SELECT `uid`,`qid` FROM A GROUP BY A.`uid`,A.`qid` HAVING COUNT(*) > 1) AND uid NOT IN (SELECT MIN(uid) FROM A GROUP BY A.`uid`,A.`qid` HAVING COUNT(*) > 1)) ; 
//删除中间表 
DELETE FROM A WHERE id IN (SELECT id FROM F); 
SELECT * FROM A;

结果

结果如下：

id	uid	qid
1	1	1
2	1	2
3	2	2
5	3	3

注：如果说不用保留一行数据的话那么就简单多了，只需要一个很简单的sql语句：

DELETE FROM A WHERE (uid,qid) IN (SELECT uid,qidFROM A GROUP BY uid,qidHAVING COUNT(*)>1)