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First posted :
03/24/2008
Times viewed :
213
XQuery Primary Expressions
XQuery basically consist of two languages: the first is XQuery, a
human-readable version, which is actually not XML, and the second is called XQueryX,
the XML version of XQuery, which is intended to be used by (automating)
applications rather than by human beings. In this tutorial however, we'll only
look at the human-readable version.
XQuery consists of a set of powerful expressions. What kind of expressions
does XQuery offer? There are Primary Expressions, Path Expressions, Sequence
Expressions, Arithmetic Expressions, Comparison Expressions, Logical
Expressions, Constructors, FLWR Expressions, Sorting Expressions, Conditional
Expressions, and Quantified Expressions, and some more.
The following sections contain worksheets that will let you enter some
example xqueries constructed using expression types, and run them over the XML
source provided earlier in this tutorial. I recommend you get the XQueryTester
up and running before continuing…
Primary Expressions
This group of expressions consists of literals, variables,
parenthesized expressions, function calls, and comments.
Each following expression type makes use of these primary expressions.
I guess that a short explanation of each of the contained expressions is in
place here. First, a literal consists of two types: numerical literals and
string literals.
Numerical literals can contain integers, decimals, and doubles. They
respectively evaluate to the data types xs:integer, xs:decimal,
and xs:double. All three types contain series of digits in conjunction
with representation methods for each data type.
String literals are a series of concatenated chars, enclosed by either single
or double quotes. Its type evaluates to xs:string. A literal of zero
characters is called an empty string.
In XQuery, variable expressions evaluate to the value of their QName, which
is dependent of the evaluation context. XQuery allows static and dynamic
variables to be used. Variables must be declared before they are used. More
information on the term QName may be found at: http://www.w3.org/TR/REC-xml-names/#NT-QName
Parenthesized expressions are simply expressions contained in double quotes.
Function calls contain a QName (i.e., a variable which evaluates to a
function calls’ name) and a parenthesized list of one or more expressions.
Currently, unfortunately the MS XQuery Demo does not yet support any function
call other than document().
Finally, comments are simply text strings enclosed in xqueries and are
ignored by the XQuery processor.
No explicit examples are included here since these expression types are in
fact the basic building blocks for use in all other XQuery Expression types that
follow. So hold on, and you’ll soon see some basic xqueries appearing, that
consist of combinations of the primary expressions mentioned above.
