I have table which has 4 columns and thousands of records. I want to delete a rows which are duplicate based on three column(say col1,col2,col3).
I have to delete row based on criteria like

```
1) found a row which has col1 and col2 equal and
2) in those duplicate record i have to delete those rows on which col3 is null- keep  one row
  (if no other record with combination of col1 and col2 doesn't have not null col3) 

col1 col2 col3
---- ---- ----
1    1    null
1    1    null
1    2    null
1    2    1
1    2    1
1    2    null
1    2    2
1    2    3

```

In this example i want to delete either 1 or 2. 3,4,5,6 has duplicate columns in and some of them has third column is not null and i want to keep only one row which has third column not null(if third column also equal keep only one row)