SQL query to detect and delete duplicate records

If no primary key, try something like this :

SELECT DISTINCT firstname, lastname
INTO #temp_table
FROM source_table

DELETE FROM source_table

INSERT INTO source_table
SELECT * FROM #temp_table

do you have a case sensistivity problem ?

if so convert both names to upper case and then do the test....

Agree with lowfat--if you have something you can use for a key, you're better off to use that method.  If not, you need to use a method like BillAn1 suggested (I would just truncate the table instead of deleting the old rows--also, if you have a lot of data, this can be very slow) or a cursor.

Here is a sample using a cursor but that does not require a separate table or a dump/reload:



DECLARE dupsCsr CURSOR READ_ONLY FOR
SELECT [firstName], [lastName], COUNT(*) AS numDups
FROM yourTable
GROUP BY [firstName], [lastName]
HAVING COUNT(*) > 1
DECLARE @firstName VARCHAR(30) --Change to match datatype on your table
DECLARE @lastName VARCHAR(30) --Change to match datatype on your table
DECLARE @numDups INT

OPEN dupsCsr
FETCH NEXT FROM dupsCsr INTO @firstName, @lastName, @numDups
WHILE @@FETCH_STATUS = 0
BEGIN
      SET @numDups = @numDups - 1 --delete all but 1 of the duplicates
      SET ROWCOUNT @numDups
      DELETE FROM yourTable
      WHERE [firstName] = @firstName AND [lastName] = @lastName
      FETCH NEXT FROM dupsCsr INTO @firstName, @lastName, @numDups
END --WHILE
CLOSE dupsCsr
DEALLOCATE dupsCsr

SET ROWCOUNT 0 --restore default