End of XML

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End of XML

• February 19th, 2003

2
FLWR (Flower) Expressions

• FOR ...
• LET...
• WHERE...
• RETURN...

3
XQuery

• Find book titles by the coauthors of Database
  Theory

FOR x IN bib/booktitle/text() Database
Theory/author y IN bib/bookauthor/tex
t() x/text()/title RETURN ltanswergt
y/text() lt/answergt
Result ltanswergt abc lt/ answer gt lt answer gt
def lt/ answer gt lt answer gt ghi lt/ answer gt
The answer willcontain duplicates !
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XQuery

• Same as before, but eliminate duplicates

FOR x IN bib/booktitle/text() Database
Theory/author y IN distinct(bib/booka
uthor/text() x/text()/title) RETURN ltanswergt
y/text() lt/answergt
Result ltanswergt abc lt/ answer gt lt answer gt
def lt/ answer gt lt answer gt ghi lt/ answer gt
distinct a function that eliminates duplicates
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SQL and XQuery Side-by-side
Product(pid, name, maker)Company(cid, name, city)
Find all products made in Seattle
FOR x in /db/Product/row y in
/db/Company/rowWHERE x/maker/text()y/cid/
text() and y/city/text() SeattleRETURN
x/name
SELECT x.nameFROM Product x, Company yWHERE
x.makery.cid and y.citySeattle
SQL
XQuery
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XQuery Nesting

• For each author of a book by Morgan Kaufmann,
  list all books she published

FOR a IN distinct(document("bib.xml")
/bib/bookpublisherMorgan
Kaufmann/author) RETURN ltresultgt
a, FOR t IN
/bib/bookauthora/title
RETURN t
lt/resultgt
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XQuery
Result

• ltresultgt
• ltauthorgtJoneslt/authorgt
• lttitlegt abc lt/titlegt
• lttitlegt def lt/titlegt
• lt/resultgt
• ltresultgt
• ltauthorgt Smith lt/authorgt
• lttitlegt ghi lt/titlegt
• lt/resultgt

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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

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XQuery
ltbig_publishersgt FOR p IN
distinct(document("bib.xml")//publisher)
LET b document("bib.xml")/bookpublisher
p WHERE count(b) gt 100 RETURN
p lt/big_publishersgt
count a (aggregate) function that returns the
number of elms
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XQuery

• Find books whose price is larger than average

LET aavg(document("bib.xml")/bib/book/price) FOR
b in document("bib.xml")/bib/book WHERE
b/price gt a RETURN b
Lets try to write this in SQL
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XQuery

• Summary
• FOR-LET-WHERE-RETURN FLWR

FOR/LET Clauses
List of tuples
WHERE Clause
List of tuples
RETURN Clause
Instance of Xquery data model
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FOR v.s. LET

• FOR
• Binds node variables ? iteration
• LET
• Binds collection variables ? one value

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FOR v.s. LET
Returns ltresultgt ltbookgt...lt/bookgtlt/resultgt
ltresultgt ltbookgt...lt/bookgtlt/resultgt ltresultgt
ltbookgt...lt/bookgtlt/resultgt ...
FOR x IN document("bib.xml")/bib/book RETURN
ltresultgt x lt/resultgt
LET x IN document("bib.xml")/bib/book RETURN
ltresultgt x lt/resultgt
Returns ltresultgt ltbookgt...lt/bookgt
ltbookgt...lt/bookgt
ltbookgt...lt/bookgt ... lt/resultgt
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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...)

Returns ltresultgt ltauthorgt...lt/authorgt
ltauthorgt...lt/authorgt
ltauthorgt...lt/authorgt
... lt/resultgt
RETURN ltresultgt b/author lt/resultgt
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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 !!

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Sorting in XQuery
ltpublisher_listgt FOR p IN distinct(document("
bib.xml")//publisher) RETURN ltpublishergt
ltnamegt p/text() lt/namegt ,
FOR b IN document("bib.xml")//bookpublisher
p RETURN ltbookgt

b/title ,
b/price

lt/bookgt SORTBY(price DESCENDING)
lt/publishergt SORTBY(name)
lt/publisher_listgt
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If-Then-Else
FOR h IN //holding RETURN ltholdinggt
h/title,
IF h/_at_type "Journal"
THEN h/editor
ELSE h/author
lt/holdinggt 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
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Universal Quantifiers
FOR b IN //book WHERE EVERY p IN b//para
SATISFIES contains(p,
"sailing") RETURN b/title
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The Role of XML Data

• XML is designed for data exchange, not to replace
  relational or E/R data
• Sources of XML data
• Created manually with text editors not really
  data
• Generated automatically from relational data
  (will discuss next)
• Text files, replacing older data formats Web
  server logs, scientific data (biological,
  astronomical)
• Stored/processed in native XML engines very few
  applications need that today

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XML from/to Relational Data

• XML publishing
• relational data ? XML
• XML storage
• XML ? relational data

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XML Publishing

• Exporting the data is easy we do this already
  for HTML
• Translating XQuery ? SQL is hard
• XML publishing systems
• Research Experanto (IBM/DB2), SilkRoute (ATT
  Labs and UW)
• XQuery ? SQL
• Commercial SQL Server, Oracle
• only Xpath ? SQL and with restrictions

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XML Publishing

• How do we choose the output structure ?
• Determined by agreement
• Or dictated by committees
• XML dialects (called applications) DTDs
• XML Data is often nested, irregular, etc
• No normal forms for XML

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XML Storage

• Most often the XML data is small
• E.g. a SOAP message
• Parsed directly into the application (DOM API)
• Sometimes XML data is large
• need to store/process it in a database
• The XML storage problem
• How do we choose the schema of the database ?

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XML Storage

• Two solutions
• Schema derived from DTD
• Storing XML as a graph Edge relation