Interfacing Relational Databases to the Web

We have already completed the first step of the design process—deciding what we need our application to do. What remains is defining the data that our application will access in three iterations:

Generally, one initially models databases in an entity-relationship diagram, which represents each table as an entity with a set of attributes, and a “join”, or query, spanning multiple tables is represented by a relationship. Even if you're not going to go to the trouble of making up an E-R diagram, you should at least consider these concepts; we shall examine the entities and relationships for the address book in prose.

The low-level model we shall be using is the relational model, which has been around since 1970 and is the model used by most commercial relational databases, including Oracle, Sybase and Informix.

Finally, we shall define the ways in which our users can see the data. Once we've completed this, we're done with the hard part and can move on to the tedious part—the implementation.

Since the entity-relationship model is such a high-level model, some of the hairier issues of the low-level model are not yet apparent, and our data model looks quite simple. The main advantage of the E-R model is that it presents a clear picture of the database miniworld—the segment of the real world that we're modeling—and is easily comprehensible to both laypersons and software engineers. Entities are defined as follows:

Relationships among entities are defined in this way:

Now we need to move our high-level model (which people can understand) to a low-level model (which our database management system can understand). This process is quite straightforward, although the database designer still has some options at this point, involving normalization. Normalization (see Resources) is a formal process to quantify and measure the quality of a relational model; as always, the tradeoff is theoretical quality versus performance.

The quick-and-dirty algorithm for moving your data model from the entity-relationship to the relational model is this:

This is necessarily a simplified version of the process; it does not deal with m:n relationships or some other details. For a more complete discussion of data model conversion, see a textbook on database theory.

SQL code for the “finished” relational model can be found at ftp.linuxjournal.com/pub/lj/listings/issue67/3475.tgz. All code is released under the GNU GPL.