Maxxus PRO SPK-23 Especificaciones descargar pdf (Pagina 202)

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written by two coauthors, each of whom had written other books, on their own or possibly with

other authors. This type of relationship usually gets a table all to itself, so you might have Books,

Authors, and Books_Authors. This third table would only contain the keys of the other tables as

foreign keys in pairs, to show which authors have been involved with which books.

How to Design Your Web Database

Knowing when you need a new table and what the key should be can be something of an art.

You can read huge reams of information about entity relationship diagrams and database nor-

malization, which are beyond the scope of this book. Most of the time, however, you can fol-

low a few basic design principles. Let’s consider these in the context of Book-O-Rama.

Think About the Real World Objects You Are Modeling

When you create a database, you are usually modeling real-world items and relationships and

storing information about those objects and relationships.

Generally, each class of real-world objects you model will need its own table. Think about it:

We want to store the same information about all our customers. If there is a set of data that has

the same “shape,” we can easily create a table corresponding to that data.

In the Book-O-Rama example, we want to store information about our customers, the books

that we sell, and details of the orders. The customers all have a name and address. The orders

have a date, a total amount, and a set of books that were ordered. The books have an ISBN, an

author, a title, and a price.

This suggests we need at least three tables in this database: Customers, Orders, and Books.

This initial schema is shown in Figure 7.3.

At present, we can’t tell from the model which books were ordered in each order. We will deal

with this in a minute.

Avoid Storing Redundant Data

Earlier, we asked the question: “Why not just store Julie Smith’s address in the Orders table?”

If Julie orders from Book-O-Rama on a number of occasions, which we hope she will, we will

end up storing her data multiple times. You might end up with an Orders table that looks like

the one shown in Figure 7.4.

There are two basic problems with this.

The first is that it’s a waste of space. Why store Julie’s details three times if we only have to

store them once?

Using MySQL

ART II

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