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Learning PHP, MySQL, and JavaScript
book

Learning PHP, MySQL, and JavaScript

by Robin Nixon
July 2009
Beginner content levelBeginner
526 pages
14h 24m
English
O'Reilly Media, Inc.
Content preview from Learning PHP, MySQL, and JavaScript

Relationships

MySQL is called a relational database management system because its tables store not only data but the relationships among the data. There are three categories of relationships.

One-to-One

A one-to-one relationship is like a (traditional) marriage: each item has a relationship to only one item of the other type. This is surprisingly rare. For instance, an author can write multiple books, a book can have multiple authors, and even an address can be associated with multiple customers. Perhaps the best example in this chapter so far of a one-to-one relationship is the relationship between the name of a state and its two-character abbreviation.

However, for the sake of argument, let’s assume that there can ever be only one customer at any address. In such a case, the Customers-Addresses relationship in Figure 9-1 is a one-to-one relationship: only one customer lives at each address and each address can have only one customer.

Usually, when two items have a one-to-one relationship, you just include them as columns in the same table. There are two reasons for splitting them into separate tables:

  • You want to be prepared in case the relationship changes later.

  • The table has a lot of columns and you think that performance or maintenance would be improved by splitting it.

The Customers table, , split into two tables
Figure 9-1. The Customers table, Table 9-8, split into two tables

Of course, when you come to build your own databases ...

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Publisher Resources

ISBN: 9780596803605Supplemental ContentErrata Page