Deleting duplicate record from table - SQL query

Question

I need to delete duplicate rows only from the table, like I have 3 duplicate rows in the table, my query will delete 2 rows from 3 duplicated rows.

How can I get this? P

Answer 1

This works in SQL Server although it isn't a single statement:

Declare @cnt int; 
Select @cnt=COUNT(*) From DupTable Where (Col1=1);  -- Assumes you are trying to delete the duplicates where some condition (e.g. Col1=1) is true.
Delete Top (@cnt-1) From DupTable

It also doesn't require any extra assumptions (like the existance of another column that makes each row unique). After all, Santanu did say that the rows were duplicates and not just the one column.

However, the right answer, in my view, is to get a real table structure. That is, add an IDENTITY column to this table so that you can use a single SQL command to do your work. Like this:

ALTER TABLE dbo.DupTable ADD
    IDCol int NOT NULL IDENTITY (1, 1)
GO

Then the delete is trivial:

DELETE FROM DupTable WHERE IDCol NOT IN 
   (SELECT MAX(IDCol) FROM DupTable GROUP BY Col1, Col2, Col3)