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CIS 550

CIS 550. Handout 7 -- XPATH and Quilt. XPath. Primary goal = to permit to access some nodes from a given document XPath main construct : axis navigation An XPath path consists of one or more navigation steps, separated by /

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CIS 550

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  1. CIS 550 Handout 7 -- XPATH and Quilt

  2. XPath • Primary goal = to permit to access some nodes from a given document • XPath main construct : axis navigation • An XPath path consists of one or more navigation steps, separated by / • A navigation step is a triplet: axis + node-test + list of predicates • Examples • /descendant::node()/child::author • /descendant::node()/child::author[parent/attribute::booktitle = “XML”][2] • XPath also offers some shortcuts • no axis means child • // º /descendant-or-self::node()/ • XPath/XSL-T quickref http://www.mulberrytech.com/quickref/index.html

  3. context node aaa aaa ccc ccc aaa 2 3 1 bbb bbb 4 5 6 7 XPath- child axis navigation • author is shorthand for child::author. Examples: • aaa -- all the child nodes labeled aaa (1,3) • aaa/bbb -- all the bbb grandchildren of aaa children (4) • */bbb all the bbb grandchildren of any child (4,6) • . -- the context node • / -- the root node

  4. XPath- child axis navigation (cont) • /doc -- all the doc children of the root • ./aaa -- all the aaa children of the context node (equivalent to aaa) • text() -- all the text children of the context node • node() -- all the children of the context node (includes text and attribute nodes) • .. -- parent of the context node • .// -- the context node and all its descendants • // -- the root node and all its descendants • //para-- all the para nodes in the document • //text() -- all the text nodes in the document • @font the font attribute node of the context node

  5. Predicates • [2] -- the second child node of the context node • chapter[5] -- the fifth chapter child of the context node • [last()] -- the last child node of the context node • chapter[title=“introduction”] -- the chapter children of the context node that have one or more title children whose string-value is “introduction” (the string-value is the concatenation of all the text on descendant text nodes) • person[.//firstname = “joe”] -- the person children of the context node that have in their descendants a firstname element with string-value “Joe” • From the XPath specification: NOTE: If $x is bound to a node set then $x = “foo” does not mean the same as not ($x != “foo”) ...

  6. Unions of Path Expressions • employee consultant -- the union of the employee and consultant nodes that are children of the context node • For some reason person/(employeeconsultant) --as in regular path expressions -- is not allowed • However person/node()[boolean(employeeconsultant)] is allowed!! • From the XPATH specification: • The boolean function converts its argument to a boolean as follows: • a number is true if and only if it is neither positive or negative zero nor NaN • a node-set is true if and only if it is non-empty • a string is true if and only if its length is non-zero • an object of a type other than the four basic types is converted to a boolean in a way that is dependent on that type

  7. Axis navigation • So far, nearly all our expressions have moved us down the by moving to child nodes. Exceptions were • . -- stay where you are • / go to the root • // all descendants of the root • .// all descendants of the context node • All other expressions have been abbreviations for child::… e.g. child::para. child:is an example of an axis • XPath has several axes: ancestor, ancestor-or-self, attribute, child, descendant, descendant-or-self, following, following-sibling, namespace, parent, preceding, preceding-sibling, self • Some of these (self, parent) describe single nodes, others describe sequences of nodes.

  8. XPath Navigation Axes(merci, Arnaud) ancestor preceding-sibling following-sibling self child attribute preceding following namespace descendant

  9. XPath abbreviated syntax (nothing) child:: @ attribute:: // /descendant-or-self::node() . self::node() .// descendant-or-self::node .. parent::node() / (document root)

  10. XPath • Reasonably widely adopted -- in XML-Schema and query languages. • Neither more expressive nor less expressive than regular path expressions (can’t do (ab)* ) • Particularly messy in some areas: • defining order of results • overloading of operations, • e.g. [chapter/title = “Introduction”] • why not [ “Introduction” IN chapter/title] ?

  11. Quilt proposed by Chamberlin, Robbie and Florescu (from the authors’ slides) • Leverage the most effective features of several existing and proposed query languages • Design a small, clean, implementable language • Cover the functionality required by all the XML Query use cases in a single language • Write queries that fit on a slide • Design a quilt, not a camel

  12. Quilt/Kweelt URLs Quilt (the language) http://www.almaden.ibm.com/cs/people/chamberlin/quilt_lncs.pdf Kweelt (the implementation) http://db.cis.upenn.edu/Kweelt/ http://db.cis.upenn.edu/Kweelt/useCases (examples in these notes)

  13. bind variables where <pattern> in <XML-expression> <pattern> in <XML-expression> … <condition> construct <expression> use variables bind variables for x in <XPath-expression> y in <XPath-expression> … where <condition> return <expression> use variables Quilt = XPath + “comprehension” syntax • XML -QL • Quilt

  14. Examples of Quilt(from http://db.cis.upenn.edu/Kweelt/useCases/R/Q1.qlt ) Relational data -- two DTDs: <?xml version="1.0" ?> <!DOCTYPE items [ <!ELEMENT items (item_tuple*)> <!ELEMENT item_tuple (itemno, description, offered_by, start_date?, end_date?, reserve_price? )> <!ELEMENT itemno (#PCDATA)> <!ELEMENT description (#PCDATA)> <!ELEMENT offered_by (#PCDATA)> <!ELEMENT start_date (#PCDATA)> <!ELEMENT end_date (#PCDATA)> <!ELEMENT reserve_price (#PCDATA)> ]> <?xml version="1.0" ?> <!DOCTYPE bids [ <!ELEMENT bids (bid_tuple*)> <!ELEMENT bid_tuple (userid, itemno, bid, bid_date)> <!ELEMENT userid (#PCDATA)> <!ELEMENT itemno (#PCDATA)> <!ELEMENT bid (#PCDATA)> <!ELEMENT bid_date (#PCDATA)> ]>

  15. The data <items> <item_tuple> <itemno>1001</itemno> <description>Red Bicycle</description> <offered_by>U01</offered_by> <start_date>1999-01-05</start_date> <end_date>1999-01-20</end_date> <reserve_price>40</reserve_price> </item_tuple> <item_tuple> <itemno>1002</itemno> <description>Motorcycle</description> <offered_by>U02</offered_by> <start_date>1999-02-11</start_date> <end_date>1999-03-15</end_date> <reserve_price>500</reserve_price> </item_tuple> … </items> <bids> <bid_tuple> <userid>U02</userid> <itemno>1001</itemno> <bid>35</bid> <bid_date>99-01-07</bid_date> </bid_tuple> <bid_tuple> <userid>U04</userid> <itemno>1001</itemno> <bid>40</bid> <bid_date>99-01-08</bid_date> </bid_tuple> … </bids>

  16. Query 1 FUNCTION date() { "1999-02-01" } <result> ( FOR $i IN document("items.xml")//item_tuple WHERE $i/start_date LEQ date() AND $i/end_date GEQ date() AND contains($i/description, "Bicycle") RETURN <item_tuple> $i/itemno , $i/description </item_tuple> SORTBY (itemno) ) </result> simple function definitions XPath expressions inorange dates are formatted so that lexicographic ordering gives the right result

  17. Output from Q1 <?xml version="1.0" ?> <result> <item_tuple> <itemno> 1003 </itemno> <description> Old Bicycle </description> </item_tuple> <item_tuple> <itemno> 1007 </itemno> <description> Racing Bicycle </description> </item_tuple> </result>

  18. Query Q2 For all bicycles, list the item number, description, and highest bid (if any), ordered by item number. <result> ( FOR $i IN document("items.xml")//item_tuple LET $b := document("bids.xml")//bid_tuple[itemno = $i/itemno] WHERE contains($i/description, "Bicycle") RETURN <item_tuple> $i/itemno , $i/description , IF ($b) THEN <high_bid> NumFormat("#####.##", max(-1, $b/bid)) </high_bid> ELSE "" </item_tuple> SORTBY (itemno) ) </result> use of variable in Xpath lots of coercion

  19. Output from Q2 <result> <item_tuple> <itemno> 1001 </itemno> <description> Red Bicycle </description> <high_bid> 55 </high_bid> </item_tuple> <item_tuple> <itemno> 1003 </itemno> <description> Old Bicycle </description> <high_bid> 20 </high_bid> </item_tuple> <item_tuple> <itemno> 1007 </itemno> <description> Racing Bicycle </description> <high_bid> 225 </high_bid> </item_tuple> <item_tuple> <itemno> 1008 </itemno> <description> Broken Bicycle </description> </item_tuple> </result>

  20. Query Q3 Find cases where a user with a rating worse (alphabetically greater than "C" ) offers an item with a reserve price of more than 1000. <result> ( FOR $u IN document("users.xml")//user_tuple, $i IN document("items.xml")//item_tuple WHERE $u/rating GT 'C' AND $i/reserve_price GT 1000 AND $i/offered_by = $u/userid RETURN <warning> <user_name>$u/name/text()</user_name>, <user_rating>$u/rating/text()</user_rating>, <item_description>$i/description/text()</item_description>, $i/reserve_price </warning> ) </result> Comparing sets with singletons Same rules as in XPath? In this case the DTD gives uniqueness

  21. Quilt -- Attributes and IDs <census> <person name = "Bill" job = "Teacher"> <person name = "Joe" job = "Painter" spouse = "Martha"> <person name = "Sam" job = "Nurse"> <person name = "Fred" job = "Senator" spouse = "Jane"> </person> </person> <person name = "Karen" job = "Doctor" spouse = "Steve"> </person> </person> <person name = "Mary" job = "Pilot"> <person name = "Susan" job = "Pilot" spouse = "Dave"> </person> </person> </person> <person name = "Frank" job = "Writer"> <person name = "Martha" job = "Programmer" spouse = "Joe"> <person name = "Dave" job = "Athlete" spouse = "Susan"> </person> </person> ... </person> </census> <?xml version="1.0" ?> <!DOCTYPE census [ <!ELEMENT census (person*)> <!ELEMENT person (person*)> <!ATTLIST person name ID #REQUIRED spouse IDREF #IMPLIED job CDATA #IMPLIED > ]>

  22. Query Q1 Find Martha's spouse: FOR $m IN document("census.xml")//person[@name="Martha"] RETURN shallow($m/@spouse->{person@name}) A hack. Kweelt does not read the DTD The shallow function strips an element of its subelements. Dereferencing

  23. Query Q6 Find Bill's grandchildren. <result> ( FOR $b IN document("census.xml")//person[@name = "Bill"] , $c IN $b/person | $b/@spouse->{person@name}/person , $g IN $c/person | $c/@spouse->{person@name}/person RETURN shallow($g) ) </result>

  24. Status of XML types • DTDs -- widely used, but limited • lack of base types • untyped pointers (IDs and IDREFs) • no tuple types (hence no record subtyping or inheritance) • XML-schema -- lots of hoopla, but • not stable • too complex • Others: RDF (not really types for XML) SOX, Relax, Schematron • Opinions: • None of these is good for database design. • Something new is needed (some core of XML-schema)

  25. Status of XML Query languages • None of them are really typed (by a DTD or anything else). • Type errors show up as empty answers • XML-QL probably the most elegant, but too powerful. • XSL and descendants are working (in IE 5) • Quilt -- nice extension of XPath, but XPath is quite complex. • Nothing like an “algebra” for any of these (though some ideas are now emerging) • Nothing like database optimization yet exists. • Do we need something simpler?

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