sql server data display change

Hi,

I have a data in a table like

Retailer_SK             Regional_SK          National_SK

12                                226                    NULL

23                                 224                   Nulll

23                                Null                    224

40                                Null                    245

40                                224                     Null

I am trying to write a query to display the data below which i am stuck

Retailer_SK            Regional_Sk           National_SK

12                                226                    NULL

23                                224                    224

40                                224                    245

Any help?????