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a database are the elements. The tables are subsets. Relationships are defined in

terms of unions and intersections of the subsets (tables).

To explain how to use a database, we will create one for automobile expenses. In the

process, we will be explaining how a database works.

Planning a database

The first step in creating a database is to ask yourself many questions. Write them

down, and leave some space between the questions to later write the answers. At

least some of the answers should seem obvious after you take some time to think.

You may have to go through this process a few times before everything becomes clear

in your mind and on paper. Using a text document for these questions and answers

makes it easier to move the questions around, add additional questions, or change

the answers.

Here are some of the questions and answers I developed before I created a database

for automobile expenses. I had an idea of what I wanted before I started, but as I

began asking questions and listing the answers, I discovered that I needed additional

tables and fields.

What are the fields going to be? My expenses divided into three broad areas: fuel

purchases, maintenance, and vacations. The annual cost for the car’s license plate

and driver’s license every four years did not fit into any of these. It will be a table of

its own: license fees.

What fields fit the fuel purchases area? Date purchased, odometer reading, fuel cost,

fuel quantity, and payment method fit. (Fuel economy can be calculated with a query.)

What fields fit the maintenance area? Date of service, odometer reading, type of

service, cost of service, and next scheduled service of this type (for example, for oil

changes list when the next oil change should be). But it would be nice if there was a

way to write notes. So, a field for notes was added to the list.

What fields fit the vacations area? Date, odometer reading, fuel (including all the

fields of the fuel table), food (including meals and snacks), motel, total tolls, and

miscellaneous. Since these purchases are made by one of two bank cards or with

cash, I want a field to state which payment type was used for each item.

What fields fit into the food category? Breakfast, lunch, supper, and snacks seem to

fit. Do I list all the snacks individually or list the total cost for snacks for the day? I

chose to divide snacks into two fields: number of snacks and total cost of snacks. I

also need a payment type for each of these: breakfast, lunch, supper, and total cost of

snacks.

What are the fields that are common to more than one area? Date appears in all of

the areas as does odometer reading and payment type.

How will I use this information about these three fields? While on vacation, I want the

expenses for each day to be listed together. The date fields suggest a relationship

between the vacation table and the dates in each of these tables: fuel and food, This

means that the date fields in these tables will be linked as we create the database.

The type of payment includes two bank cards and cash. So, we will create a table

with a field for the type of payment and use it in list boxes in the forms.

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