It depends on the type of the field and how well the data has been conntrolled.
I would have thought that the field would be a Yes/No or numeric field controlled by a tickbox, in which case it would be:
Code:
SELECT * FROM Table WHERE PaidFor = 0
otherwise if it is a string or date field it would either be:
Code:
SELECT * FROM Table WHERE PaidFor IS NULL
or if some of the fields contain empty strings (ie. it is not null, but still empty) then you would need
Code:
SELECT * FROM Table WHERE PaidFor IS NULL or PaidFor = ''