Argee's Access Blog

In this simple example, we have been developing a database to assist with the design of ice cream sundaes.

In the last article in this series, we developed the statement of purpose for the database:

This database stores information about the quantity of contents used to make ice cream sundaes. It assists with the design of new sundaes and provides reports of the ingredients required to make each type of sundae.

We also determined that we will need at least four tables if the database is to be properly structured:

• ingredients
• dishes (containers)
• sizes
• sundaes

I am going to save deciding the contents of each of these tables for the next article and focus now on how the tables relate to each other. (After all, we want this to be a relational database.)

The process to determine the type of relationship that exists between any two tables is quite straight-forward and perhaps a bit tedious. Try not to let the tediousness of the task of getting in the way of doing it. Correctly defining the database's relationships is absolutely essential!!

Unlike real-world relationships, in Access there are always exactly two partners in any relationship. You define each relationship by considering one pair of tables. By filling in the blanks in two simple statements, you can make a simple conclusion about the type of the relationship.

So we are going to look at pairs of tables until we have determined what the relationship is between each possible pair of tables using these three statements.

• Each record in table A can have (zero/one/many) records in table B
• Each record in table B can have (zero/one/many) records in table A
• Therefore this is a (one to many/one to one/many to many) relationship.

So here are the pairs of tables and their relationships.

With four tables there are six possible relationships but not all tables will share a relationship.

sundaes <---> dishes

(Just to be clear, the dishes table stores information about types of dishes so the relationship definition deals with how each sundae will relate to dish types.)

• Each sundae can have one dish
• Each dish can have many sundaes
• Therefore this is a one to many relationship

sundaes <---> sizes

• Each sundae can have many sizes
• Each size can have many sundaes
• Therefore this is a many to many relationship

sundaes <---> ingredients

• Each sundae can have many ingredients
• Each ingredient can be used in many sundaes
• Therefore this is a many to many relationship

ingredients <---> dishes

• Each ingredient can have no dishes
• Each dish can have no ingredients
• Therefore there is no relationship between ingredients and dishes

ingredients <---> sizes

• Each ingredient has no records in the sizes table
• Each size has no records in the ingredients table
• Therefore there is no relationship between ingredients and sizes

dishes <---> sizes

• Each dish has one size
• Each size could be applied to many dishes
• Therefore there is a one to many between sizes and dishes.

Remember that when you are describing relationships, you are not defining the relationship. A relationship is what it is; calling it by another name will not change its essential nature. Your job is to identify accurately the essential nature of each relationship in the database.

A couple of things have come to light in this relationship analysis. First, sizes are used in two somewhat different contexts. Sundae sizes refer to the size (small, medium, large) in which the sundae will be marketed. Dish sizes on the other hand refer to the capacity of the dish, how large a sundae it can comfortably contain. The two uses may or may not similar descriptions. A sundae might be described as small medium or large. So might a dish but dishes could alternatively be described in terms of units of measure (grams, ounces, etc.)

So it might be worthwhile thinking about an additional table to store the possible units of measure (metrics.) Whether this would be necessary or merely a possibly good idea would depend in part on how many possible units of measures apply.

Including a metrics table requires further relationship analysis. Before we can describe the relationship between sundaes and metrics in particular, it is important to be clear what constitutes a sundae. So far it looks as if we have defined a sundae as the combination of ingredients and a particular container. But there is one more factor that distinguishes one sundae from another: size.

So a banana split consists of vanilla, chocolate, and strawberry ice cream, banana, chocolate syrup, crushed pineapple, walnuts, whipped cream, and cherries, regardless of whether the sundae is small medium or large. For data management purposes, a small banana split is different from a medium or large banana split and it is the size that distinguishes them from each other.

metrics <---> sundaes

• Each metric applies to many sundaes
• Each sundae has one metric
• Therefore this is a one to many relationship

metrics <---> ingredients

• Each ingredient has many metrics
• Each metric apples to many ingredients
• Therefore this is a many to many relationship

metrics <---> dishes

• Each metric applies to many dishes
• Each dish has one metric
• Therefore this is a one to many relationship

metrics <---> sizes

• Each metric applies to many sizes
• Each size has one metric
• Therefore this is a one to many relationship

Now, you might be asking how can a metric apply to many dishes. That is a perfectly legitimate question simply because I haven't fully defined what I mean by a dish.

For this definition, let's look to the real world as it applies to ice cream shop. We have already seen that dishes come in several sizes. In my shop, I intend to have both edible and inedible dishes for each of the sizes. Perhaps, in the future, I might have several types of edible dishes (waffle or chocolate, for example.) It is the combination of size and dish type that distinguishes one small dish from another.

Raising that point means that we must think about yet another set of relationships. I'm going to leave that decision aside for the moment.

Next time we will take a look at table definitions, what fields each of our tables need.