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Chapter 2. Database Design

Once you install your DBMS software on your computer, it can be very tempting to just jump right into creating a database without much thought or planning. As with any software development, this kind of ad hoc approach works with only the simplest of problems. If you expect your database to support any kind of complexity, some planning and design will definitely save you time in the long run. You will need to take a look at what details are important to good database design.

2.1. Database Design

Suppose you have a large collection of compact discs and you want to create a database to track them. The first step is to determine what the data that you are going to store is about. One good way to start is to think about why you want to store the data in the first place. In our case, we most likely want to be able to look up CDs by artist, title, and song. Since we want to look up those items, we know they must be included in the database. In addition, it is often useful to simply list items that should be tracked. One possible list might include: CD title, record label, band name, song title. As a starting point, we will store the data in the table shown in Table 2-1.

Table 2-1. A CD Database Made Up of a Single Table

Band Name

CD Title

Record Label

Songs

Stevie Wonder

Talking Book

Motown

You Are the Sunshine of My Life, Maybe Your Baby, Superstition, . . .

Miles Davis Quintet

Miles Smiles

Columbia

Orbits, Circle, . . .

Wayne Shorter

Speak No Evil

Blue Note

Witch Hunt, Fee-Fi-Fo-Fum

Herbie Hancock

Headhunters

Columbia

Man, . . .

Herbie Hancock

Maiden Voyage

Blue Note

Maiden Voyage

(For brevity's sake, we have left out most of the songs.) At first glance, this table seems like it will meet our needs since we are storing all of the data we need. Upon closer inspection, however, we find several problems. Take the example of Herbie Hancock. "Band Name" is repeated twice: once for each CD. This repetition is a problem for several reasons. First, when entering data in the database, we end up typing the same name over and over. Second, and more important, if any of the data changes, we have to update it in multiple places. For example, what if "Herbie" were misspelled? We would have to update the data in each of the two rows. The same problem would occur if the name Herbie Hancock changes in the future (à la Jefferson Airplane or John Cougar). As we add more Herbie Hancock CDs to our collection, we add to the amount of effort required to maintain data consistency.

Another problem with the single CD table lies in the way it stores songs. We are storing them in the CD table as a list of songs in a single column. We will run into all sorts of problems if we want to use this data meaningfully. Imagine having to enter and maintain that list. And what if we want to store the length of the songs as well? What if we want to perform a search by song title? It quickly becomes clear that storing the songs in this fashion is undesirable.

This is where database design comes into play. One of the main purposes of database design is to eliminate redundancy from the database. To accomplish this task, we use a technique called normalization. Before we start with normalization, let's start with some fundamental relational database concepts. A data model is a diagram that illustrates your database design. It is made up of three main elements: entities, attributes, and relationships. For now, let's focus on entities and attributes; we will take a look at relationships later.



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