概述
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a2,a3判断(实际使用中可以拓展为多个)
实现过程
1. 创建表A,数据如下
a1 | a2 | a3 |
---|---|---|
1 | 1 | 1 |
2 | 1 | 2 |
3 | 2 | 2 |
4 | 2 | 2 |
5 | 3 | 3 |
6 | 2 | 2 |
2. 选择重复的行:
select *,count(*)
from A group by a2,a3
having count(*)>1;
a1 | a2 | a3 | count(*) |
---|---|---|---|
3 | 2 | 2 | 3 |
使用in来找到我们想要的ID
SELECT *
FROM A
WHERE (a2,a3) IN
(SELECT A.`a2`,A.`a3`
FROM A
GROUP BY A.`a2`,A.`a3`
HAVING COUNT(*)>1)
a1 | a2 | a3 |
---|---|---|
3 | 2 | 2 |
4 | 2 | 2 |
6 | 2 | 2 |
那么后面就很好办了:
3. 选出要删除的值:
SELECT *
FROM A
WHERE (a2, a3) IN
(SELECT `a2`,`a3`
FROM A
GROUP BY A.`a2`,A.`a3`
HAVING COUNT(*) > 1)
AND a1 NOT IN
(SELECT MIN(a1)
FROM A
GROUP BY A.`a2`,A.`a3`
HAVING COUNT(*) > 1) ;
-- 结果是保留a1最小的值,其他选项全部选出,
-- 请注意此时并不是将Select 改为delete就可以了,如果你直接这样改的话,会报如下错误:
-- You can't specify target table 'A' for update in FROM clause
-- 该错误提示你,不能先select出同一表中的某些值,再update这个表(在同一语句中)。所以要稍微修改一下。
4. 删除值
sql语句如下:
//创建中间表
CREATE TABLE F(a1 INTEGER,a2 INTEGER,a3 INTEGER);
//将要删除的数据插入中间表
INSERT INTO F (
SELECT *
FROM A
WHERE (a2, a3) IN (SELECT `a2`,`a3`
FROM A GROUP BY A.`a2`,A.`a3`
HAVING COUNT(*) > 1)
AND a1 NOT IN
(SELECT MIN(a1) FROM A
GROUP BY A.`a2`,A.`a3`
HAVING COUNT(*) > 1)) ;
//删除中间表
DELETE FROM A WHERE a1 IN (SELECT a1 FROM F);
SELECT *FROM A;
a1 | a2 | a3 |
---|---|---|
1 | 1 | 1 |
2 | 1 | 2 |
3 | 2 | 2 |
5 | 3 | 3 |
备注
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 (a2,a3) IN (SELECT a2,a3 FROM A GROUP BY a2,a3 HAVING COUNT(*)>1)