MySQL Query deletes duplicate records

Source: Internet
Author: User
Tags mysql query repetition rowcount

Ways to query and delete duplicate records

1, look for redundant records in the table, duplicate records are based on a single field (Peopleid) to determine

SELECT * from people where Peopleid in (select Peopleid from People GROUP by Peopleid have count (people ID) > 1)

2, delete redundant records in the table, duplicate records are based on a single field (Peopleid) to judge, leaving only the smallest ROWID records

Delete from people where Peopleid in (select Peopleid from People GROUP by Peopleid have count (Peopleid) > 1 )

and rowID not in (select min (rowid) from people GROUP by Peopleid have Count (Peopleid) >1)

3. Find redundant duplicate records (multiple fields) in the table

SELECT * from Vitae a WHERE (A.PEOPLEID,A.SEQ) in (select Peopleid,seq to Vitae GROUP by PEOPLEID,SEQ have count (*) > 1)

4. Delete extra duplicate records (multiple fields) in the table, leaving only the record with ROWID minimum

Delete from Vitae a where (A.PEOPLEID,A.SEQ) in (select Peopleid,seq from Vitae GROUP by PEOPLEID,SEQ have count (*) ; 1)

and rowID not in (select min (rowid) from Vitae GROUP by PEOPLEID,SEQ have Count (*) >1)

5. Find redundant duplicate records (multiple fields) in the table, not including the smallest ROWID records

SELECT * from Vitae a WHERE (A.PEOPLEID,A.SEQ) in (select Peopleid,seq from Vitae GROUP by PEOPLEID,SEQ have Count (*) & Gt 1)

and rowID not in (select min (rowid) from Vitae GROUP by PEOPLEID,SEQ have Count (*) >1)

Two

For example There is a field "name" in Table A, and the "name" value between different records is likely to be the same, now it is necessary to query out the records in the table, the "name" value has duplicate entries; Select Name,count (*) from A Group by name Having Count (*) > 1

If you also look at the same gender, the following is true: Select name,sex,count (*) from A Group by Name,sex have Count (*) > 1

Three

Method One

Declare @max integer, @id integer

Declare cur_rows cursor Local for select main field, COUNT (*) from table name Group by main field having count (*) >; 1

Open Cur_rows

Fetch cur_rows into @id, @max

While @ @fetch_status =0

Begin

Select @max = @max-1

SET ROWCOUNT @max

Delete from table name where main field = @id

Fetch cur_rows into @id, @max

End

Close Cur_rows

SET ROWCOUNT 0

Method Two

There are two meanings of duplicate records, one is a completely duplicate record, that is, all fields are duplicated records, and the second is some key field duplicate records, such as the Name field repeats, and other fields may not be repeated or can be ignored.

1, for the first kind of repetition, it is easier to solve, using the SELECT DISTINCT * from TableName can be a result set without duplicate records.

If the table needs to delete duplicate records (duplicate records retain 1), you can delete them as follows

SELECT DISTINCT * to #Tmp from TableName

drop table TableName

SELECT * Into TableName from #Tmp

drop table #Tmp

This duplication occurs because the table is poorly designed and the unique index columns are added to resolve.

2, this kind of repetition problem usually requires to keep the first record in the duplicate record, the operation method is as follows

Suppose there is a duplicate field name,address, which requires the result set to be unique for both fields

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 on (select Autoid from #tmp2)

The last select is the result set that name,address not duplicate (but one more autoid field, which can be written in the SELECT clause without this column in the actual write)

Four

Duplicate query

SELECT * FROM tablename where ID in (theselect ID from the tablename GROUP by ID has a count (ID) > 1)

MySQL Query deletes duplicate records

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