Rune Bivrin (9/24/2010)
most of the time it doesn't matter why it's empty because all that's done with the contents is display it in reports and dialog boxes. In those cases NULL is just an annoying special case that adds no value.
Most of the time, but not all.
You might have a table of staff information with no cell phone column, but need to add one. You add it as a NULL column meaning "we don't know yet", and ask users to provide the information when they next log in, and which point it's updated to either the number, or an empty string if they don't have one. Or if they don't answer (the ophone is at home and they don't know the number) it's left as NULL and they're asked again next time they log in.
If you don't have NULL columns then for this example you need a Not Null cell phone column, plus a not null bit column to indicate whether the question needs to be asked again.