PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

Designing Your Web Database
211
Relationships
Foreign keys represent a relationship between data in two tables. For example, the link
from Orders to Customers represents a relationship between a row in the Orders table
and a row in the Customers table.
Three basic kinds of relationships exist in a relational database.They are classified
according to the number of elements on each side of the relationship. Relationships can
be either one-to-one, one-to-many, or many-to-many.
A one-to-one relationship means that one of each thing is used in the relationship.
For example, if you put addresses in a separate table from Customers, they would have a
one-to-one relationship between them.You could have a foreign key from Addresses to
Customers or the other way around (both are not required).
In a one-to-many relationship, one row in one table is linked to many rows in another
table. In this example, one Customer might place many Orders. In these relationships,
the table that contains the many rows has a foreign key to the table with the one row.
Here, we put the CustomerID into the Order table to show the relationship.
In a many-to-many relationship, many rows in one table are linked to many rows in
another table. For example, if you have two tables, Books and Authors, you might find
that one book was 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 contain only the keys of the other tables as foreign keys in pairs, to show which
authors are involved with which books.
Designing 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 reams of information about entity relationship diagrams and database
normalization, which are beyond the scope of this book. Most of the time, however, you
can follow a few basic design principles. Let’s consider them 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 needs its own table.Think about
it: You want to store the same information about all your customers. If a set of data has
the same “shape,” you can easily create a table corresponding to that data.
In the Book-O-Rama example, you want to store information about customers, the
books that you sell, and details of the orders.The customers all have names and addresses.
Each order has a date, a total amount, and a set of books that were ordered. Each book
has an International Standard Book Number (ISBN), an author, a title, and a price.