XML Demystified

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How XQuery Works
XML Demystifi ed
An XQuery must contain a conditional expression that specifies the search criteria.
A conditional expression is either true or false. For example, /catalog/cd/title =
‘XML Demystified’ is a conditional expression. Saxon-B looks at the next title tag
in the XML document and determines if the text of the title tag is ‘XML Demystified’.
If so, then the conditional expression is true; otherwise, the conditional expression
is false.
A conditional expression is used within a FLWOR expression. FLWOR sounds
like more gibberish, but it’s really an acronym for for, let, where, order by, and
return clauses. A clause is a component of an XQuery.
For, Let, and Order By Clauses
You use the for and let clauses to assign values to variables within the XQuery. A
variable is a placeholder for a value such as $x. Look at the catalog.xq file and
you’ll notice the following for clause. For each title tag in the catalog XML file,
Saxon-B assigns the text of the current title XML tag to the $x variable and then
sorts the titles.
for $x in doc("catalog.xml")/catalog/cd/title
Look at the code that follows the for clause and you’ll see that variable $x is used
by the order by clause. The text of the title replaces the $x variable when Saxon-B
runs the XQuery before processing the order by clause.
The order by clause places the value of $x in sorted order. In this example, the
order by clause sorts all the titles in alphabetical order before storing the titles into
the output.html file. Titles within the XML document remain unchanged.
You can specify the direction of the sort by using ascending or descending, as we
show here. The default direction is ascending.
order by $x descending
The let clause, not used in the catalog.xq example, assigns a value to a variable.
Suppose you want to assign the title ‘XML Demystified’ to variable $x. Here’s how
you write the let clause to do this. You can then use $x in place of ‘XML Demystified’
throughout the XQuery.
let $x := 'XML Demystified'
The Where and Return Clauses
You use the where clause to specify a filter criterion using a conditional expression.
Let’s say that you want to see titles by Jimi Hendrix. You can use the where clause