Querying XML (cont.)

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?

Querying XML Data XPath = simple navigation through the tree XQuery = the SQL of XML XSLT = recursive traversal –will not discuss in class

Sample Data for Queries Addison-Wesley Serge Abiteboul Rick Hull Victor Vianu Foundations of Databases 1995 Freeman Jeffrey D. Ullman Principles of Database and Knowledge Base Systems 1998 Addison-Wesley Serge Abiteboul Rick Hull Victor Vianu Foundations of Databases 1995 Freeman Jeffrey D. Ullman Principles of Database and Knowledge Base Systems 1998

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

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

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

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 { $y/text() } FOR $x IN bib/book[title/text() = “Database Theory”]/author $y IN bib/book[author/text() = $x/text()]/title RETURN { $y/text() } Result: abc def ghi The answer will contain duplicates !

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 { $y/text() } FOR $x IN bib/book[title/text() = “Database Theory”]/author $y IN distinct(bib/book[author/text() = $x/text()]/title) RETURN { $y/text() } Result: abc def ghi distinct = a function that eliminates duplicates

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 { $a, FOR $t IN /bib/book[author=$a]/title RETURN $t } FOR $a IN distinct(document("bib.xml") /bib/book[publisher=“Morgan Kaufmann”]/author) RETURN { $a, FOR $t IN /bib/book[author=$a]/title RETURN $t }

XQuery Jones abc def Smith ghi Jones abc def Smith ghi Result:

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

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

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 $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…

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

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

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

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 { $b/author } Returns:...

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 !!

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

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

Existential 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 SOME $p IN $b//para SATISFIES contains($p, "sailing") AND contains($p, "windsurfing") RETURN { $b/title }

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

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 ?

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

XML from/to Relational Data XML publishing: –relational data  XML XML storage: –XML  relational data

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

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

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 ?

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