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XQuery
CPS 116
Introduction to Database Systems
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Announcements
Homework #3 will be assigned this Thursday
(October 21)
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XQuery
XPath + full-fledged SQL-like query language
XQuery expressions can be
XPath expressions
FLWR (C) expressions
Quantified expressions
Aggregation, sorting, and more…
An XQuery expression returns a result XML
documents
Compare with an XPath expression, which returns a
node-set or an atomic value (boolean, number, string)
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A simple XQuery based on XPath
Find all books with price lower than $50
<result>
{
document(“bib.xml”)/bibliography/book[@price<50]
}
</result>
Things outside {}’s are copied to output verbatim
Things inside {}’s are evaluated and replaced by the results
document(“bib.xml”) specifies the document to query
The XPath expression returns a set of book elements
These elements (including all their descendents) are copied to
output
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FLWR expressions
Retrieve the titles of books published before 2000,
together with their publisher
<result>{
for $b in document(“bib.xml”)/bibliography/b ook
let $p := $b/publisher
where $b/year < 2000
return
<book>
{ $b/title }
{ $p }
</book>
}</result>
for: loop
$b ranges over the result node-set,
getting one node at a time
let: assignment
$p gets the entire result of
$b/publisher (possibly many nodes)
where: filter condition
return: result structuring
Invoked in the “innermost loop,” i.e.,
once for each successful binding of all
query variables
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An equivalent formulation
Retrieve the titles of books published before 2000,
together with their publisher
<result>{
for $b in
return
<book>
{ $b/title }
{ $b/publisher }
</book>
}</result>
document(“bib.xml”)/bibliography/book[year<2 000]
