Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Chapter 16: A Brief XML Primer
The contents of the Note elements, such as “The customer called … ” are neither an element nor an
attribute, yet they are valid XML data.
Be aware that such data exists in XML, but SQL Server will not output data in this format using any of
the automatic styling methods. The row/column approach of RDBMS systems lends itself far better to
elements and attributes. To output data such as our notes, you would need to use some of the methods
that allow for more explicit output formats of your XML (and these can be non-intuitive at best) or perform
a transformation on the output after you select it. We will look at transformations as the last item in
this chapter.
Being Valid vs. Being Well Formed — Schemas and DTDs
Just because an XML document is well formed does not mean that it is valid XML. Now, while that is
sinking in on you I’ll tell you that no XML is considered “valid” unless it has been validated against
some form of specification document. Currently, there are only two recognized types of specification
documents — a Document Type Definition, or DTD, and an XML schema.
The basic premise behind both varieties of validation documents is much the same. While XML as a language
defines the most basic rules that a XML document must comply with, DTDs and XML schemas
seek to define what the rules are for a particular class of XML document. The two approaches are implemented
somewhat differently and each offers distinct advantages over the other:
❑ DTDs: This is the old tried and true. DTDs are utilized in SGML (XML is an SGML application
— you can think of SGML being a superset of XML, but incredibly painful to learn), and
have the advantage of being a very well-known and accepted way of doing things. There are
tons of DTDs already out there that are just waiting for you to utilize them.
The downside (you knew there had to be one — right?) is that the “old” is operative in my “old
tried and true” statement. Not that being old is a bad thing, but in this case, DTDs are definitely
not up to speed with what else has happened in document technology. DTDs do not really allow
for such seemingly rudimentary things as restricting data types. You’ll find that DTDs — at least
in terms of being used with XML — are largely being treated as deprecated at this point in favor
of XML schemas.
❑ XML schemas: XML schemas have the distinct advantage of being strongly typed. What’s cool
about them is that you can effectively establish your own complex data types — types that are
made up based on combinations of one or more other data types (including other complex data
types) or require specialized pattern matching (for example, a Social Security number is just a
number, but it has special formatting that you could easily enforce via an XML schema). XML
schemas also have the advantage, as their name suggests, of being an XML document themselves.
This means that a lot of the skills in writing your XML documents also apply to writing
schemas (though there’s still plenty to learn) and that schemas can, themselves, be self-describing
— right down to validating themselves against yet another schema.
What SQL Ser ver Brings to the P arty
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So, now we have all the basics of what XML is down. What we need is to understand the relevance in
SQL Server.