My assumption is that
net_amount should always be a number, and always equal to
(total_amount - ifnull(fee_amount, 0)).
Is there ever a time where
net_amount should be null, and if so, what's the rationale for such a case?
For background, I'm finding many older contributions where
net_amount IS NULL (in a civicrm installation which was recently upgraded a from version 4.2.17) and am considering corrective action, because this situation makes for incorrect values in the "Net" statistics on contribution reports.