Delete “duplicate” rows in SQL Server 2010

Question

I made a mistake in a bulk insert script, so now i have "duplicate" rows with different colX. I need to delete this duplicate rows, but I cant figure out how. To be more precise, I have this:

 col1 | col2 | col3 | colX      
----+----------------------
  0   |  1   |  2   |  a
  0   |  1   |  2   |  b
  0   |  1   |  2   |  c
  0   |  1   |  2   |  a
  3   |  4   |  5   |  x
  3   |  4   |  5   |  y
  3   |  4   |  5   |  x
  3   |  4   |  5   |  z

and I want to keep the first occurrence of each (row, colX):

 col1 | col2 | col3 | colX      
----+----------------------
  0   |  1   |  2   |  a
  3   |  4   |  5   |  x

Thank you for your replies :)

Answer 1

If you are OK with just keeping the minimum value of colX, you can do this:

delete t from t inner join 
    (select  min(colx) mincolx, col1, col2, col3
     from t
     group by col1, col2, col3
     having count(1) > 1) as duplicates
   on (duplicates.col1 = t.col1
   and duplicates.col2 = t.col2
   and duplicates.col3 = t.col3
   and duplicates.mincolx <> t.colx)

The problem is that you still have rows where all four columns are the same. To get rid of these, after running the first query, you then have to use a temp table.

SELECT distinct col1, col2, col3, colx 
INTO temp
  FROM (SELECT col1, col2, col3
         from t 
         group by col1, col2, col3
         having count(1) > 1) subq;

DELETE from t where exists 
   (select 1 from temp 
     where temp.col1 = t.col1 
       and temp.col2 = t.col2 
       and temp.col3 = t.col3);

Here's an example SQLFiddle.