Sorry, can you spell out what you want - it isn't completely clear.

Suppose you have a data source with:

ID,column1,column2,column3
1,row1col1,row1col2,row1col3
2,row2col1,row2col2,row2col3
3,,row3col2,row2col3
4,,,row4col3

(maybe you could invent rows if you need them to demonstate what you want)

and a mail merge main document with

{ MERGEFIELD column1 }

what do you want to happen when Word is processing the row with ID=1? with
ID=3?

Peter Jamieson