Several ways to delete duplicate data in SQL Server

During the use of the database, due to the problems of the program, there are sometimes duplicate data, which causes the database to be set up incorrectly.

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 duplicates of the record, one is a completely duplicate record, that is, all the fields are duplicate records, the second is some key fields duplicate records, such as the Name field repeat, and other fields do not necessarily repeat or repeat can be ignored.

1, for the first repetition, easier to solve, the use of

SELECT DISTINCT * FROM tablename

You can get a result set without duplicate records.

If the table needs to delete duplicate records (1 of duplicate records are retained), you can delete them in the following ways

SELECT DISTINCT * into #Tmp tablename

DROP TABLE TableName

SELECT * INTO TableName from #Tmp

drop table #Tmp

This duplication occurs because the table is poorly designed and can be resolved by adding a unique index column.

2. This type of repetition usually requires the first record in the duplicate record to be retained, as follows

Suppose there is a duplicate field of name,address that requires a unique result set 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 in (select Autoid from #tmp2)

The last select gets the name,address result set (but one more autoid field, which can be written in the SELECT clause to omit this column)