Part I: Relational Database Design

   

---

A Word About the Examples and Techniques in This Book

You'll notice that there are a wide variety of examples in this book. I've made sure that they are as generic and relevant as possible. However, you may notice that several of the examples are rather simplified, incomplete, or even on occasion incorrect. Believe it or not, I created them that way on purpose.

I've created some examples with errors so that I could illustrate specific concepts and techniques. Without these examples, you wouldn't see how the concepts or techniques are put to use, as well as the results you should expect from using them. Other examples are simple because, once again, the focus is on the technique or concept and not on the example itself. For instance, there are many ways that you can design an order-tracking database. However, the structure of the sample order-tracking database I use in this book is simple because the focus is specifically on the design process, not on creating an elaborate order-tracking database system.

So what I'm really trying to emphasize here is this:

Focus on the concept or technique and its intended results, not on the example used to illustrate it.

A New Approach to Learning

Here's an approach to learning the design process (or pretty much anything else, for that matter) that I've found very useful in my database-design classes.

Think of all the techniques used in the design process as a set of tools; each tool (or technique) is used for a specific purpose. The idea here is that once you learn generically how a tool is used, you can then use that tool in any number of situations. The reason you can do this is because you use the tool the same way in each situation.

Take a Crescent wrench, for example. Generically speaking, you use a Crescent wrench to fasten and unfasten a nut to a bolt. You open or close the jaw of the wrench to fit a given bolt by using the adjusting screw located on the head of the wrench. Now that you have that clear, try it out on a few bolts. Try it on the legs of an outdoor chair , or the valve cover on an engine, or the side panel of an outdoor cooling unit, or the hinge plates of an iron gate. Do you notice that regardless of where you encounter a nut and bolt, you can always fasten and unfasten the nut by using the Crescent wrench in the same manner?

The tools used to design a database work in exactly the same way. Once you understand how a tool is used generically, it will work the same way regardless of the circumstances under which it is used. For instance, consider the tool (or technique) for decomposing a field value. Say you have a single A DDRESS field in a CUSTOMERS table that contains the street address, city, state, and zip code for a given customer. You'll find it difficult to use this field in your database because it contains more than one item of data; you'll certainly have a hard time retrieving information for a particular city or sorting the information by a specific zip code.

The solution to this apparent dilemma is to decompose the A DDRESS field into smaller fields. You do this by identifying the distinct items that make up the value of the field, and then treating each item as its own separate field. That's all there is to it! This process constitutes a "tool" that you can now use on any field containing a value composed of two or more distinct data items, such as these sample fields. Figure I.1 shows the results of the decomposition process.

Figure I.1. Decomposing fields containing multiple data items.

Note

You'll learn more about decomposing field values in Chapter 7, "Establishing Table Structures."

You can use all of the techniques ("tools") that are part of the design process presented in this book in the same manner. You'll be able to design a sound database structure using these techniques regardless of the type of database you need to create. Just be sure to remember this:

Focus on the concept or technique being presented and its intended results, not on the example used to illustrate it.

---

Top