Q on default assignment to a column at INSERT time

I have a table called "drawing" with the following columns drawing.mslink [serial] drawing.a [char(5)] drawing.b [char(3)] drawing.c [char(20)] The problem is, at INSERT time, I want the engine to concatenate the columns a and b and assign it to column c for the same record. I know for the default assignment to work at UPDATE time a TRIGGER does the trick but I am not sure how to do it at INSERT time. The default assignment desired at INSERT time is drawing.c = drawing.a || drawing.b If you could kindly post the solution here or e-mail me at aiyahv@aecl.ca it would be highly appreciated Thanks Vish