End of XML

1. 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$xINbib/book[title/text() = “Database Theory”]/author$yINbib/book[author/text() = $x/text()]/title RETURN <answer> { $y/text() } </answer> Result: <answer> abc </ answer > < answer > def </ answer > < answer > ghi </ answer > The answer willcontain duplicates !

4. XQuery Same as before, but eliminate duplicates: FOR$xINbib/book[title/text() = “Database Theory”]/author$yINdistinct(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

5. FOR$y in /db/Company/row[city/text()=“Seattle”]$x in /db/Product/row[maker/text()=$y/cid/text()]RETURN { $x/name } CoolXQuery SQL and XQuery Side-by-side Product(pid, name, maker)Company(cid, name, city) Find all products made in Seattle FOR$x in /db/Product/row$y in /db/Company/rowWHERE$x/maker/text()=$y/cid/text() and $y/city/text() = “Seattle”RETURN { $x/name } SELECT x.nameFROM Product x, Company yWHERE x.maker=y.cid and y.city=“Seattle” SQL XQuery

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

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

8. 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

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

10. 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…

11. 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

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

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

14. 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: <result> <author>...</author> <author>...</author> <author>...</author> ... </result> RETURN <result> { $b/author } </result>

15. 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 !!

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

17. If-Then-Else FOR$h IN //holding RETURN <holding> { $h/title, IF$h/@type = "Journal" THEN$h/editor ELSE$h/author } </holding> SORTBY (title)

18. Existential Quantifiers FOR$b IN //book WHERESOME$p IN $b//paraSATISFIES contains($p, "sailing") AND contains($p, "windsurfing") RETURN { $b/title }

19. Universal Quantifiers FOR$b IN //book WHEREEVERY$p IN $b//paraSATISFIES contains($p, "sailing") RETURN { $b/title }

20. 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

21. XML from/to Relational Data • XML publishing: • relational data  XML • XML storage: • XML  relational data

22. 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 (AT&T Labs and UW) • XQuery  SQL • Commercial: SQL Server, Oracle • only Xpath  SQL and with restrictions

23. 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

24. 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 ?

25. XML Storage Two solutions: • Schema derived from DTD • Storing XML as a graph: “Edge relation”