If you want C to always be A-B then it would, in my opinion, be a very
bad idea to store C in the table. Instead you can just put C in your query.
SELECT `A`, `B`, `A` - `B` AS `C` FROM `table`
If that seems like a hassle, you could always create a view using that
> For simplicity's sake, let's say I have three fields, A, B and C, all
> of which are integers. I'd like the value of C to be equal to A less B
> (A-B). Is there a way I can perform this calculation? I'm guessing it
> would happen when I INSERT a row and specify the values for A and B.
> Feel free to direct me to the fine manual I should have read.
> Thank you.