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If we have more than one customer, that’s not a problem — we just add another customer node:
Indeed, this can go to unlimited levels of hierarchy (subject, of course, to whatever your parser can handle).
We could, for example, add a level for individual line items in the order.
Determining Elements vs. Attributes
Chapter 16: A Brief XML Primer
The first thing to understand here is that there is no hard and fast rule for determining what should be
an element vs. an attribute. An attribute describes the properties of the element that it is an attribute of.
Child elements — or child “nodes” — of an element do much the same thing. So how, then, do we decide
which should be which? Why are attributes even necessary? Well, like most things in life, it’s something
of a balancing act.
Attributes make a lot of sense in situations where the value is a one-to-one relationship with, and is
inherently part of, the element. In AdventureWorks2008, for example, we have only one customer number
per customer ID — this is ideal for an attribute. As we are transforming our relational data to XML,
the columns of a table will often make good attributes to an element directly related to individual rows
of a table.
Elements tend to make more sense if there is more of a one-to-many relationship between the element
and what’s describing it. In our example earlier in the chapter, there are many sales orders for each customer.
Technically speaking, we could have had each order be an attribute of a customer element, but
then we would have needed to repeat much of the customer element information over and over again.
Similarly, if our AdventureWorks2008 database allowed for the notion of customers having aliases (multiple
account numbers — similar to how they have multiple contacts), then we may have wanted to have an
AccountNumber element under the customer and have its attribute describe individual instances of names.
Whichever way you go here, stick to one rule I’ve emphasized many times throughout the book — be
consistent. Once something of a given nature is defined as being an attribute in one place, lean toward
keeping it an attribute in other places unless its nature is somehow different in the new place you’re
using it. One more time: Be consistent.
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