SQL去重
1 SQL去重
SQL中去除完全相同数据可以用distinct关键字，任意字段去重可以用group by，以下面的数据表为例。
2 distinct
存在两条完全相同的纪录，用关键字distinct就可以去掉
根据单个字段去重，能精确去重;
作用在多个字段时，只有当这几个字段的完全相同时，才能去重;
关键字distinct只能放在SQL语句中的第一个，才会起作用
一般用来返回不重复的记录条数，返回不重复的条数(去掉test重复的，就剩下6条)
; 3 group by
1. 查询根据名字去重后数据(名字相同取id值大的)
SELECT * FROM stu WHERE id IN (SELECT MAX(id) FROM stu GROUP BY name)
2. 删除名字相同数据(名字相同保留id值大的)
group by + count + max去掉重复数据
1）SELECT * FROM stu
2）加上group by 后，会将重复的数据去掉了
3) 条件(名字)是数量大于1的重复数据
SELECT name FROM stu GROUP BY name HAVING COUNT(name) > 1
#条件是数量大于1的重复数据
SELECT * FROM stu WHERE name IN(
SELECT name FROM stu GROUP BY name HAVING COUNT(name)>1
)
4）查看某字段重复数据的id
SELECT id, COUNT(*) FROM stu
GROUP BY NAME DESC HAVING(COUNT(*) > 0)
5）查询所有重复数据
SELECT * FROM stu WHERE NAME IN (SELECT name FROM stu GROUP BY name HAVING COUNT(name) > 1)
5) 去重
可以使用distinct去重（返回不重复的用户名）
删除多余的重复记录（name），只保留id最大的记录。
DELETE FROM stu
WHERE id NOT IN ( SELECT a.id FROM ( SELECT MAX( id ) AS id FROM stu GROUP BY name )a )
或者
DELETE FROM stu WHERE name IN (SELECT name FROM (SELECT name FROM stu GROUP BY name HAVING COUNT(name)>1) e)
AND id NOT IN (SELECT id FROM (SELECT MAX(id) AS id FROM stu GROUP BY name HAVING COUNT(name)>1) t)
#查询显示重复的数据都是显示最前面的几条，因此不需要查询是否最小值
错误删除
DELETE FROM stu WHERE name IN (SELECT name FROM stu GROUP BY name HAVING COUNT(name)>1)
AND id NOT IN (SELECT MAX(id) FROM stu GROUP BY stu HAVING COUNT(name)>1)
原因是：不能将直接查处来的数据当做删除数据的条件，我们应该先把查出来的数据新建一个临时表，然后再把临时表作为条件进行删除功能
4 总结
- 去重后名字记录
SELECT name FROM stu
GROUP BY NAME HAVING(COUNT(*) > 0)
2）
所有重复名字的记录
SELECT name FROM stu
GROUP BY NAME HAVING COUNT(*) > 1
3）把所有重复的记录都删了
DELETE FROM stu WHERE name IN
(SELECT name FROM stu GROUP BY name HAVING COUNT(*)>1)
无法在删除时同时查询这张表，这个问题只在MySQL中出现，oracle没有。怎么解决？我们只需要在查出结果以后加一张中间表。让执行器认为我们要查的数据不是来自正在删的这张表就可以了。
DELETE FROM stu WHERE name IN
(SELECT a.name FROM
(SELECT name FROM stu GROUP BY name HAVING COUNT(*)>1) a)
所有重复数据都删除, 就剩王五一条数据了
4) 现在删除所有重复数据数据做完了，考虑怎么保留重复数据中id最小的。只需要在删除时让删除该条的记录id不在重复数据id最小的当中就可以了。
DELETE FROM stu WHERE name IN
(SELECT a.name FROM
(SELECT name FROM stu GROUP BY name HAVING COUNT(*)>1) a)
AND id NOT IN
(SELECT b.id FROM
(SELECT MIN(id) id FROM stu
GROUP BY name HAVING COUNT(*)>1) b);
还有简单办法 算出去重后所有数据（保留最小ID），然后删除id不在该数组里的
DELETE FROM stu WHERE id NOT IN (SELECT t.id FROM (SELECT MIN(id) AS id FROM stu GROUP BY name)t)
Original: https://blog.csdn.net/qq_37705525/article/details/124812776
Author: qq_37705525
Title: SQL如何删除重复数据
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