Datagrid Update

Hi, I have a datagrid which is populated using a dataadapter. The Problem here is that if some of the fields are blank, it displays NULL there. How do i avoid that. If i try to to use a sql statement like command.commandtext = "Select fld1, fld2, NVL(Fld3,' '), fld4 from Table. then the blank value gets supressed, but in this case, my dataadapter.update fails. It does not update my table. So the way I made it to update is by running through all the records in the datagrid, and updating it back to the table based on the primary key by using update command which I think has a bad performance when compared to dataadapter.update. Any comments ...... Vishal