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Introduction to XQuery (Part 4 of 4)

Learn to use XQuery conditional and quantified expressions to filter and select data, and to modify query results by including custom functions.




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his is the final part of the four part series on Xquery. In this final section, you'll see an explanation of conditional and quantified expressions as well as a quick tutorial on writing your own functions in XQuery.

Writing Conditional Expressions
As you would expect, XQuery provides an if-then-else clause that lets you branch execution based on a condition. The syntax of the if-then-else is:

if ( test-expression ) then result-expression1 else result-expression2

For example, the following query checks whether customer 1001 ordered any item with a price over 200.

# Query listing -- XQuery41.ixq in samples.zip let $price := document("data/PO.xml")//po[customer/custno= '1001']/lineitems/lineitem/item/price[. > 200 ] return if ( count($price) ) then <result> customer ordered an expensive item! </result> else <result> No expensive item was ordered. </result>

The variable $price represents a sequence of price items whose value is greater than 200. The if-then-else expression displays one of two different messages depending on the result (true or false) of evaluating the test expression count($price).

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