I am very new to databases and so this is likely a dumb question.  I have a
spreadsheet that I am trying to migrate to a DB.  The problem is that I don't
know how to re-create the calculation that I have in my spreadsheet.

It's pretty simple stuff, I have a "gross", "expense" and a "revenue'
column.  In my spreadsheet, I use a simple forumla to automatically populate
my "revenue" column (revenue = gross-expense).

I am setting up a web interface that will use the database as it's source.  
I'm hoping to have my sales reps enter thier sales data into a asp-page and
have the "revenue" column calculated automatically instead of them entering
it manually.  Does this make sense?  I suspect I will need to build a
function or a query of some sort, but am struggling to find answers.  thanks
in adavance for the help.