Xpath & xquery

Xpath: Wildcard
Result: <first-name> Rick </first-name>
<last-name> Hull </last-name>
* Matches any element
//author/*
3

Xpath: Attribute Nodes
Result: “55”
@price means that price is an attribute
/bib/book/@price
4

Xpath: Predicates
Result: <author> <first-name> Rick </first-name>
<last-name> Hull </last-name>
</author>
/bib/book/author[firstname]
5

Xpath: More Predicates
Result: <lastname> … </lastname>
<lastname> … </lastname>
/bib/book/author[firstname][address[//zip][city]]/lastname
6

Xpath: More Predicates
/bib/book[@price < “60”]
/bib/book[author/@age < “25”]
/bib/book[author/text()]
7

Xpath: Summary
bib matches a bib element
* matches any element
/ matches the root element
/bib matches a bib element under root
bib/paper matches a paper in bib
bib//paper matches a paper in bib, at any depth
//paper matches a paper at any depth
paper|book matches a paper or a book
@price matches a price attribute
bib/book/@price matches price attribute in book, in bib
bib/book/[@price<“55”]/author/lastname matches…
8

Comments on XPath?
• What’s good about it?
• What can’t it do that you want it to do?
• How does it compare, say, to SQL?
9

XQuery
• Based on Quilt, which is based on XML-QL
• Uses XPath to express more complex queries
10

FLWR (“Flower”) Expressions
FOR ...
LET...
WHERE...
RETURN...
11

XQuery
Find all book titles published after 1995:
FOR $x IN document("bib.xml")/bib/book
WHERE $x/year > 1995
RETURN { $x/title }
Result:
<title> abc </title>
<title> def </title>
<title> ghi </title>
12

XQuery
Find book titles by the coauthors of “Database Theory”:
FOR $x IN bib/book[title/text() = “Database Theory”]/author
$y IN bib/book[author/text() = $x/text()]/title
RETURN <answer> { $y/text() } </answer>
Result:
<answer> abc </ answer >
< answer > def </ answer >
< answer > ghi </ answer >
The answer will
contain duplicates !
13

XQuery
Same as before, but eliminate duplicates:
FOR $x IN bib/book[title/text() = “Database Theory”]/author
$y IN distinct(bib/book[author/text() = $x/text()]/title)
RETURN <answer> { $y/text() } </answer>
Result:
<answer> abc </ answer >
< answer > def </ answer >
< answer > ghi </ answer >
distinct = a function
that eliminates duplicates
14

XQuery: Nesting
For each author of a book by Morgan Kaufmann, list all
books she published:
FOR $a IN distinct(document("bib.xml")
/bib/book[publisher=“Morgan Kaufmann”]/author)
RETURN <result>
{ $a,
FOR $t IN /bib/book[author=$a]/title
RETURN $t
}
</result>
15

XQuery
<result>
<author>Jones</author>
<title> abc </title>
<title> def </title>
</result>
<result>
<author> Smith </author>
<title> ghi </title>
</result>
Result:
16

XQuery
• FOR $x in expr -- binds $x to each value in the list expr
• LET $x = expr -- binds $x to the entire list expr
• Useful for common subexpressions and for aggregations
17

XQuery
count = a (aggregate) function that returns the number of elms
<big_publishers>
FOR $p IN distinct(document("bib.xml")//publisher)
LET $b := document("bib.xml")/book[publisher = $p]
WHERE count($b) > 100
RETURN { $p }
</big_publishers>
18

XQuery
Find books whose price is larger than average:
LET $a=avg(document("bib.xml")/bib/book/price)
FOR $b in document("bib.xml")/bib/book
WHERE $b/price > $a
RETURN { $b }
Let’s try to write this in SQL…
19

XQuery
Summary:
• FOR-LET-WHERE-RETURN = FLWR
FOR/LET Clauses
WHERE Clause
RETURN Clause
List of tuples
List of tuples
Instance of Xquery data model
20

FOR v.s. LET
FOR
• Binds node variables  iteration
LET
• Binds collection variables  one value
21

FOR v.s. LET
FOR $x IN document("bib.xml")/bib/book
RETURN <result> { $x } </result>
Returns:
<result> <book>...</book></result>
...
LET $x IN document("bib.xml")/bib/book
RETURN <result> { $x } </result>
Returns:
<result> <book>...</book>
<book>...</book>
<book>...</book>
...
</result>
22

Collections in XQuery
• Ordered and unordered collections
• /bib/book/author = an ordered collection
• Distinct(/bib/book/author) = an unordered
collection
• LET $a = /bib/book  $a is a collection
• $b/author  a collection (several authors...)
RETURN <result> { $b/author } </result>
Returns:
<result> <author>...</author>
<author>...</author>
<author>...</author>
...
</result>
23

Collections in XQuery
What about collections in expressions ?
• $b/price  list of n prices
• $b/price * 0.7  list of n numbers
• $b/price * $b/quantity  list of n x m numbers ??
• $b/price * ($b/quant1 + $b/quant2) 
$b/price * $b/quant1 + $b/price * $b/quant2 !!
24

Sorting in XQuery
<publisher_list>
FOR $p IN distinct(document("bib.xml")//publisher)
RETURN <publisher> <name> { $p/text() } </name> ,
FOR $b IN document("bib.xml")//book[publisher = $p]
RETURN <book>
{ $b/title ,
$b/price
}
</book> SORTBY(price DESCENDING)
</publisher> SORTBY(name)
</publisher_list>
25

If-Then-Else
FOR $h IN //holding
RETURN <holding>
{ $h/title,
IF $h/@type = "Journal"
THEN $h/editor
ELSE $h/author
}
</holding> SORTBY (title)
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Existential Quantifiers
FOR $b IN //book
WHERE SOME $p IN $b//para SATISFIES
contains($p, "sailing")
AND contains($p, "windsurfing")
RETURN { $b/title }
27

Universal Quantifiers
FOR $b IN //book
WHERE EVERY $p IN $b//para SATISFIES
contains($p, "sailing")
RETURN { $b/title }
28

Other Stuff in XQuery
• BEFORE and AFTER
• for dealing with order in the input
• FILTER
• deletes some edges in the result tree
• Recursive functions
• Currently: arbitrary recursion
• Perhaps more restrictions in the future ?
29

Prepared by:-
Meghavi Patel 30

Xpath & xquery

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Xpath & xquery