Friday, March 28, 2008

Delete Duplicate rows from the table.

Delete Duplicate rows from the table.
Suppose there is a table called "EmployeeTable" which have some duplicate records.
There is a three way to delete the duplicate rows.

First way to delete duplicate rows :


Select distinct * into Emp_Temp_Table from EmployeeTable

In the above line we are inserting all the distinct row of the "EmployeeTable" to another table "Emp_Temp_Table" (Emp_Temp_Table will create automatically when you use the above query.)
Actuall the above query create clone of EmployeeTable and insert all the distinct row inside the Clone Table (Emp_Temp_Table).


drop table EmployeeTable

sp_rename 'Emp_Temp_Table',EmployeeTable'

Then Delete the orginal table and rename the clone table with the name of orginal table.


Second way to delete duplicate rows :

Select distinct * into Emp_Temp_Table from EmployeeTable

Truncate table EmployeeTable

insert into EmployeeTable select * from Emp_Temp_Table

drop table Emp_Temp_Table


Third way to delete duplicate rows :

Populate the new Primary Key


Alter table EmployeeTable add NewPK int NULL
Go
Declare @intCounter int
Set @intCounter = 0
Update EmployeeTable
SET @intCounter = NewPK = @intCounter + 1

Select name,RecCount=count(*), PktoKeep = max(NewPK)
Into #dupes
From EmployeeTable
Group by name
Having count(*) > 1
Order by count(*) desc


Delete dupes except one Primary key for each dup record


Delete test
from EmployeeTable a join #dupes d
a.name
where a.NewPK not in (select PKtoKeep from #dupes)



Remove the NewPK column


ALTER TABLE test DROP COLUMN NewPK
go

drop table #dupes

4 comments:

Sapien said...

This is the best, no nonsense, and most simplistic answer I've seen so far. That one line created the table, with all the right columns for me, then I just renamed the old table and it's done. Thanks.

webtips said...

The below article showed me a simple way
Delete Duplicate Rows in Sql Server

-Webtips

neeraj said...

tahnks.
This is very good query.I have asked this question my time in interview.It very simple to remember.

Jagan Mohan Reddy said...

This link discribes different methods of deleting duplicate rows

http://www.besttechtools.com/SQLArt
icles.aspx?ID=DeleteDuplicate