问题描述
|
我有唯一的
id
和email
字段。电子邮件被重复。我只想保留所有重复项的一个电子邮件地址,但保留最新的“ 0”(最后插入的记录)。
我该如何实现?
解决方法
想象您的表“ 3”包含以下数据:
select id,email
from test;
ID EMAIL
---------------------- --------------------
1 aaa
2 bbb
3 ccc
4 bbb
5 ddd
6 eee
7 aaa
8 aaa
9 eee
因此,我们需要找到所有重复的电子邮件并删除所有重复的电子邮件,但是最新的ID。
在这种情况下,重复aaa
,bbb
和eee
,因此我们要删除ID 1、7、2和6。
为此,首先我们需要查找所有重复的电子邮件:
select email
from test
group by email
having count(*) > 1;
EMAIL
--------------------
aaa
bbb
eee
然后,从该数据集中,我们需要为这些重复的电子邮件中的每一封找到最新的ID:
select max(id) as lastId,email
from test
where email in (
select email
from test
group by email
having count(*) > 1
)
group by email;
LASTID EMAIL
---------------------- --------------------
8 aaa
4 bbb
9 eee
最后,我们现在可以删除所有ID小于LASTID的电子邮件。因此解决方案是:
delete test
from test
inner join (
select max(id) as lastId,email
from test
where email in (
select email
from test
group by email
having count(*) > 1
)
group by email
) duplic on duplic.email = test.email
where test.id < duplic.lastId;
我现在没有在这台机器上安装mySql,但是应该可以
更新资料
上面的删除有效,但是我发现了一个更优化的版本:
delete test
from test
inner join (
select max(id) as lastId,email
from test
group by email
having count(*) > 1) duplic on duplic.email = test.email
where test.id < duplic.lastId;
您会看到它删除了最旧的重复项,即1、7、2、6:
select * from test;
+----+-------+
| id | email |
+----+-------+
| 3 | ccc |
| 4 | bbb |
| 5 | ddd |
| 8 | aaa |
| 9 | eee |
+----+-------+
另一个版本是Rene Limon删除的内容
delete from test
where id not in (
select max(id)
from test
group by email)
,正确的方法是
DELETE FROM `tablename`
WHERE id NOT IN (
SELECT * FROM (
SELECT MAX(id) FROM tablename
GROUP BY name
)
)
,试试这个方法
DELETE t1 FROM test t1,test t2
WHERE t1.id > t2.id AND t1.email = t2.email
,我必须说,优化的版本是一段甜美,优雅的代码,即使在DATETIME列上进行比较,它的作用也很吸引人。这是我在脚本中使用的内容,我在其中搜索每个EmployeeID的最新合同结束日期:
DELETE CurrentContractData
FROM CurrentContractData
INNER JOIN (
SELECT
EmployeeID,PeriodofPerformanceStartDate,max(PeriodofPerformanceEndDate) as lastDate,ContractID
FROM CurrentContractData
GROUP BY EmployeeID
HAVING COUNT(*) > 1) Duplicate on Duplicate.EmployeeID = CurrentContractData.EmployeeID
WHERE CurrentContractData.PeriodofPerformanceEndDate < Duplicate.lastDate;
非常感谢!