The calculation won't show up on your table unless you store it there, which is (generally) poor database design. Unless it's a value you need a 'point in time' record of the value, you will be able to recalculate it in your query any time you want! An example where you might want to save the calculation: an invoice amount (price*amount) where the price might change later, but you need your invoice to be correct for auditing purposes.
Saving a calculation when it's not needed amounts to 'saving data in two places', which is inefficient and leads to data errors later on. What if you DO change the value of part of the calculation? All of a sudden your stored value is no longer valid. What if someone makes a typo and you don't discover it until later? These are just examples.
As for the rest of your question, I answered you under the post in the Queries section. Please just post in one Forum, as duplicate questions can lead to duplicate, incomplete, or contradictory answers.