Access 2007 for Starters: The Missing Manual

Calculated Fields

When you started designing tables, you learned that it's a database crime to add information that's based on the data in another field or another table. An example of this mistake is creating a Products table that has both a Price and a PriceWithTax field. The fact that the PriceWithTax field is calculated based on the Price field is a problem. Storing both is a redundant waste of space. Even worse, if the tax rate changes, then you're left with a lot of records to update and the potential for inconsistent information (like a with-tax price that's lower than a no-tax price).

Even though you know not to create fields like PriceWithTax, sometimes you will want to see calculated information in Access. Before Boutique Fudge prints a product list for one of its least-loved retailers, it likes to apply a 10 percent price markup. To do this, it needs a way to adjust the price information before printing the data. If the retailer spots the lower price without the markup, they're sure to demand it.

Queries provide the perfect solution for these kinds of problems, because they include an all-purpose way to mathematically manipulate information. The trick's to add a calculated field: a field that's defined in your query, but doesn't actually exist in the table. Instead, Access calculates the value of a calculated field based on one or more other fields in your table. The values in the calculated field are never stored anywhere—instead, Access generates them each time you run ...