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

Chapter 16: A Brief XML Primer
Note that the URL portion of the namespace declaration must be entered on a single line. They are
shown here word wrapped onto multiple lines because there is a limit to the number of characters we
can show per line in print. Make sure you include the entire URL on a single line.
Enforcing Constraints Beyond the Schema Collection
We are, of course, used to the concept of constraints by now. We’ve dealt with them extensively in this
book. Well, if our relational database needs constraints, it follows that our XML data does too. Indeed,
we’ve already implemented much of the idea of constraints in XML through the use of schema collections.
But what if we want to enforce requirements that go beyond the base schema?
Surprisingly, you cannot apply XML data type methods within a constraint declaration. How do you get
around this problem? Well, wrap the tests up in a user-defined function (UDF), and then utilize that
function in your constraint.
I have to admit I’m somewhat surprised that the methods are not usable within the CONSTRAINT declaration,
but things like functions are. All I can say is “go figure. . . .” I’ll just quietly hope they fix this
in a future release, as it seems a significant oversight on something that shouldn’t have been all that difficult
(yeah, I know — easy for me to say since they have to write that code, not me!).
Retrieving Relational Data in XML Format
Retrieving relational data is the area that SQL Server already had largely figured out even in the older
releases. We had a couple of different options, and we had still more options within those options —
between them all, things have been pretty flexible for quite some time. Let’s take a look . . . .
The FOR XML Clause
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This clause is at the root of many of the different integration models available. It is essentially just an
option added onto the end of the existing T-SQL SELECT statement, but serves as the primary method
for taking data stored in normal relational format and outputting it as XML.
Let’s look at the SELECT statement syntax from Chapter 3:
SELECT [TOP () [PERCENT] [WITH TIES]]
[FROM ]
[WHERE ]
[GROUP BY ]
[HAVING ]
[ORDER BY ]
[[FOR XML {RAW|AUTO|EXPLICIT|PATH [()]}
[, XMLDATA | XMLSCHEMA [()]]
[, ELEMENTS [XSINIL | ABSENT]][, BINARY base 64][ROOT(‘‘)]]
[OPTION (, [, ...n])]
Most of this should seem pretty trivial by now — after all, we’ve been using this syntax throughout a lot
of hard chapters by this time — but it’s time to focus in on that FOR XML line … .