MySQL之-查詢/刪除重複記錄的方法大全詳解

尋找所有重複標題的記錄：

SELECT * FROM t_info a WHERE ((SELECT COUNT(*) FROM t_info WHERE Title = a.Title) > 1) ORDER BY Title DESC

一、尋找重複記錄

1、尋找全部重複記錄

Select * From 表 Where 重复字段 In (Select 重复字段 From 表 Group By 重复字段 Having Count(*)>1)

2、過濾重複記錄(只顯示一條)

Select * From HZT Where ID In (Select Max(ID) From HZT Group By Title)

註：此處顯示ID最大一筆記錄

二、刪除重複記錄

1、刪除全部重複記錄（慎用）

Delete 表 Where 重复字段 In (Select 重复字段 From 表 Group By 重复字段 Having Count(*)>1)

2、保留一筆（這應該是大多數人所需要的^_^）

Delete HZT Where ID Not In (Select Max(ID) From HZT Group By Title)

註：此處保留ID最大一筆記錄

三、舉例

1、查找表中多餘的重複記錄，重複記錄是根據單一欄位（peopleId）來判斷

select * from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)

2、刪除表中多餘的重複記錄，重複記錄是根據單一欄位（peopleId）來判斷，只留有rowid最小的記錄

delete from people where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1) 
and rowid not in (select min(rowid) from people group by peopleId having count(peopleId )>1)

3、查找表中多餘的重複記錄（多個欄位）

select * from vitae a where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)

4、刪除表中多餘的重複記錄（多個欄位），只留有rowid最小的記錄

delete from vitae a where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1) 
and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1)

5、查找表中多餘的重複記錄（多個字段），不包含rowid最小的記錄

select * from vitae a where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1)
 and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1)

四、補充

有兩個以上的重複記錄，一是完全重複的記錄，也即所有字段均重複的記錄，二是部分關鍵欄位重複的記錄，例如Name欄位重複，而其他欄位不一定重複或都重複可以忽略。
1、對於第一種重複，比較容易解決，使用

select distinct * from tableName

就可以得到無重複記錄的結果集。
如果該表需要刪除重複的記錄（重複記錄保留1條），可以按以下方法刪除

select distinct * into #Tmp from tableName
drop table tableName
select * into tableName from #Tmp
drop table #Tmp

發生這種重複的原因是表設計不周產生的，增加唯一索引列即可解決。
2、這類重複問題通常要求保留重複記錄中的第一筆記錄，操作方法如下
假設有重複的欄位為Name,Address，要求得到這兩個欄位唯一的結果集

select identity(int,1,1) as autoID, * into #Tmp from tableName
select min(autoID) as autoID into #Tmp2 from #Tmp group by Name,autoID
select * from #Tmp where autoID in(select autoID from #tmp2)

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