XML Demystified

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XML Demystifi ed
XML developers define middle_name as having zero to one occurrence. That is,
the customer may have no middle name, or only one middle name but not multiple
middle names.
Reviewing Table 3-1, you’ll notice that the question mark is the symbol you use
to indicate zero to one occurrence. Here’s how you write this in the declaration for
the middle_name:
This declaration of the name and the middle_name enables the XML document
to contain the following data. You’ll notice two customers are entered into this
document. The first customer doesn’t have a middle name and second has one. Both
are valid. However, the first customer name will be invalid if you remove the
question mark from the middle_name in the name declaration in the DTD because
the first customer doesn’t have a middle name.
Bob
Smith
Bob
Alex
Smith
We show you a good example of zero to many in our original XML example. The
customers tag may contain zero to many customer elements. You may have a process
where you transmit a file every day, whether or not there are any new records. There
are many business cases where you’ll need a positive confirmation that there haven’t
been any new customers that day.
Now you can change the business rule a little bit and say that you only want the
file transmitted if at least one customer is defined. If this is the business rule, then
you can change the definition of the customers element so it has one to many
customer elements.
Optional Child Elements
The declaration of a parent element contains the names of child elements that are
contained within the parent, which we discussed earlier in this chapter. However, all
those child elements must be present for the XML document to be valid. If one is
missing, then the entire document is invalid.