1. Querying XML and Semistructured Data
CSE 350 Fall 2003

2. Query 1 SELECT row: X FROM biblio.X WHERE “Smith” in X.author . . .
book
paper
book
Answer = {row: {author:“Smith”,
date: 1999,
title: “Database…”}, }
&o12
&o24
&o29
. . .
author
title
date
author
title
author
date
&25
&30
&o52
1976
Roux
&96
Database
Systems
&o47
&o48
&o50
Combalusier
Smith
1999
Database
Systems

3. Query 2 SELECT author: X FROM biblio.book.author X . . .
Answer = {author: “Smith”,
author: “Roux”,
author: “Comalusier”}
&o1
book
paper
book
&o12
&o24
&o29
. . .
author
title
date
author
title
author
date
&25
&30
&o52
1976
Roux
&96
Database
Systems
&o47
&o48
&o50
Combalusier
Smith
1999
Database
Systems

4. Query 3
SELECT row: ( SELECT author: X FROM X.author Y) FROM biblio.book X
biblio
&o1
book
paper
book
Answer = {row: {author:“Smith”}, row: {author:“Roux”, author:“Combalusier”,}, }
&o12
&o24
&o29
. . .
author
title
date
author
title
author
date
&25
&30
&o52
1976
Roux
&96
Database
Systems
&o47
&o48
&o50
Combalusier
Smith
1999
Database
Systems

5. SELECT ( SELECT row: {author: Y, title: T} FROM X. author Y, X
SELECT ( SELECT row: {author: Y, title: T} FROM X.author Y, X.title T) FROM biblio.book X WHERE “Roux” in X.author
Query 4
biblio
&o1
book
Answer = {row: {author:“Roux”,
title: “Database…”}, row: {author:“Combalusier”,
title: “Database…”}, }
paper
book
&o12
&o24
&o29
. . .
author
title
date
author
title
author
date
&25
&30
&o52
1976
Roux
&96
Database
Systems
&o47
&o48
&o50
Combalusier
Smith
1999
Database
Systems

6. Semantics Given query Q and database DB
SELECT E[X1, …, Xn] FROM F WHERE C
Given query Q and database DB
Answer(Q,DB) is defined in two steps
Step 1: compute all bindings:
Cij are node oids or atomic values
Must satisfy paths in F and conditions in C
Step 2: answer is E[C11, …, C1n] U … U E[Cm1, …, Cmn]
For nested subqueries, apply semantics recursively X1 X2 … Xn
Ci1
Ci2
Cin

7. XQuery http://www.w3.org/TR/xquery (12/2001) XML Query data model
Ordered
FLWR (“Flower”) Expressions
FOR ... LET...
WHERE...
RETURN...

8. 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>

9. XQuery
For each author of a book by Morgan Kaufmann, list all books s/he 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>
distinct = a function that eliminates duplicates

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

11. XQuery Summary of the FLWR structure
FOR: Binds node variables  iteration
LET: Binds collection variables  one value
FOR/LET Clauses
WHERE Clause
RETURN Clause
List of tuples
Instance of Xquery data model

12. 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

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

14. XQuery Example: Find books whose price is larger than avg.
Example: using distinct and count
LET $a=avg(document("bib.xml")/bib/book/price)
FOR $b in document("bib.xml")/bib/book
WHERE $b/price > $a
RETURN $b
<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>

15. 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...)
$b/price  list of n prices
$b/price *  list of n numbers
RETURN <result> $b/author </result>
Returns:
<result> <author>...</author>
<author>...</author>
...
</result>

16. 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>
Sorting arguments: refer to the name space of the RETURN clause, not the FOR clause

17. More Xquery: If-Then-Else
FOR $h IN //holding
RETURN <holding>
$h/title,
IF = "Journal" THEN $h/editor
ELSE $h/author
</holding> SORTBY (title)
More Xquery: existential & universal quantifiers
FOR $b IN //book
WHERE SOME $p IN $b//para SATISFIES
contains($p, "sailing") AND contains($p, "windsurfing")
RETURN $b/title
"
FOR $b IN //book
WHERE EVERY $p IN $b//para SATISFIES
contains($p, "sailing")
RETURN $b/title

18. Other Stuff in XQuery BEFORE and AFTER FILTER
for dealing with order in the input
FILTER
deletes some edges in the result tree
No GROUPBY currently in Xquery
Some recent proposals exist
Recursive functions
Currently: arbitrary recursion
Perhaps more restrictions in the future ?

19. Lorel
Minor syntactic differences in regular path expressions (% instead of _, # instead of _*)
Existential variables:
What happens with books having multiple authors ? Author is existentially quantified:
SELECT biblio.book.year FROM biblio.book WHERE biblio.book.author = “Roux”
SELECT biblio.book.year FROM biblio.book X, X.author Y WHERE Y = “Roux”

20. UnQL Patterns: Equivalent to:
SELECT row: X WHERE {biblio.book: {author “Roux”, title X}} in DB,
SELECT row: X FROM biblio.book Y, Y.author Z, Y.title X WHERE Z=“Roux”

21. UnQL
Label variables:
“find all publication types and their titles where Roux is an author”
SELECT row: {type: L, title : Y} WHERE {biblio.L: {author “Roux”, title X}} in DB,

22. UnQL
Unrestricted use of label variables creates problems:
So, in UnQL regular path expressions cannot contain label variables:
Pat ::= Var | Const | {L1:Pat1, …, Ln:Patn} L ::= RegularPathExpression | LabelVariable
SELECT row: {type: L, title : Y} WHERE {biblio.(book|L).title X} in DB,
SELECT row: {type: L, title : Y} WHERE {biblio.(L)*.title X} in DB,