XML Demystified

34
Types of Document Type Definition
XML Demystifi ed
There are two types of document type definitions (DTD): an internal DTD and an
external DTD. An internal DTD appears at the beginning of the XML document
within the DOCTYPE tag, which we illustrate in the next example.
Internal DTD is perfect for when only one XML document will use the DTD
because the DTD is distributed in the same file as the XML document. Some
developers also use an internal DTD for small documents where the DTD is unlikely
to change and, especially, for those documents that will only be distributed within
their organization.
Avoid using an internal DTD if many XML documents share the same DTD
because it isn’t economical to replicate the DTD in every XML document. Further-
more, you’ll need to hunt down each of those documents whenever you want to
change the DTD.
Notice that the following example uses an internal DTD to define a customer for
the XML document. This DTD contains a number of parent-child relationships (see
Chapter 2), as you can tell from reading the definition of the customer tag.
The customer tag is a parent than contains seven child elements. These are name,
title, company, address, phone, email, and account_number. Three of those child
elements are also parents to their own child elements.
For example, name has first_name and last_name as child elements. Address has
street_1, street_2, city, state, and zip as child elements. And phone has business,
cell, home, and fax as child elements.
The DTD ends with ]>. The XML document begins on the next line, where tags
defined in the DTD identify data within the XML document.