From: Chris W
Date: August 21 2010 3:16pm
Subject: Re: Performing subtraction between fields
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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 
select.

Chris W

b@stripped wrote:
> Hello,
>
> 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.
>
>
>