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

216 Chapter 8 Designing Your Web Database
The first way means adding a Review column to the Books table.This way, there is a
field for the Review to be added for each book. If many books are in the database, and
the reviewer doesn’t plan to review them all, many rows won’t have a value in this
attribute.This is called having a null value.
Having many null values in your database is a bad idea. It wastes storage space and
causes problems when working out totals and other functions on numerical columns.
When a user sees a null in a table, he doesn’t know whether it’s because this attribute is
irrelevant, whether the database contains a mistake, or whether the data just hasn’t been
entered yet.
You can generally avoid problems with many nulls by using an alternate design. In
this case, you can use the second design proposed in Figure 8.7. Here, only books with a
review are listed in the Book_Reviews table, along with their reviews.
Note that this design is based on the idea of having a single in-house reviewer; that is,
a one-to-one relationship exists between Books and Reviews. If you want to include
many reviews of the same book, this would be a one-to-many relationship, and you
would need to go with the second design option. Also, with one review per book, you
can use the ISBN as the primary key in the Book_Reviews table. If you have multiple
reviews per book, you should introduce a unique identifier for each.
Summary of Table Types
You will usually find that your database design ends up consisting of two kinds of tables:
n Simple tables that describe a real-world object.They might also contain keys to
other simple objects with which they have a one-to-one or one-to-many relationship.
For example, one customer might have many orders, but an order is placed by
a single customer.Thus, you put a reference to the customer in the order.
n Linking tables that describe a many-to-many relationship between two real objects
such as the relationship between Orders and Books.These tables are often associated
with some kind of real-world transaction.
Web Database Architecture
Now that we’ve discussed the internal architecture of the database, we can look at the
external architecture of a web database system and discuss the methodology for developing
a web database system.
The basic operation of a web server is shown in Figure 8.8.This system consists of
two objects: a web browser and a web server. A communication link is required between
them. A web browser makes a request of the server.The server sends back a response.
This architecture suits a server delivering static pages well.The architecture that delivers
a database-backed website, however, is somewhat more complex.