CIS 550 Handout 7 — XPATH and XQuery

CIS 550 Handout 7 -- XPATH and XQuery CIS 550 Handout 7 Fall 2001 1

URLs -- XPath • http: //www. w 3. org/TR/xpath This is the “recommendation”. Dense. Few examples. Difficult to extract the “big picture” from the morass of detail • http: //www. zvon. org/xxl/XPath. Tutorial/ General/examples. html A tutorial with some simple examples. Maybe too simple. There are lots of tutorials on the web. CIS 550 Handout 7 Fall 2001 2

URLs -- XQuery • http: //www. w 3. org/TR/xquery/ The basic recommendation. Plenty of examples, so work through these first. • http: //www. w 3. org/TR/query-semantics/ A formal semantics for XQuery. Despite its forbidding title, it is remarkably readable. It also discusses a type system for XQuery. • http: //www. w 3. org/TR/xmlquery-use-cases A bunch of example queries and their solution in XQuery (not surprising, since XQuery is Turingcomplete!) CIS 550 Handout 7 Fall 2001 3

How to Identify nodes in a Tree -- Regular Path Expressions db depts dept mgr name In the normal syntax of regular expressions: emps db. emps. emp emp name db. _*. name db. (depts. dept. mgr |emps. emp) “Mary” “John” “Bill” N. B. Regular path expressions have nothing to do with regular expresions in DTDs CIS 550 Handout 7 Fall 2001 4

More examples With the DTD: … the regular path expression (PERSON. MOTHER)* identifies matrilineal ancestry XPATH is a “superset of a subset” of regular path expressions. (It cannot express this set of nodes. ) However, it is not limited to moving “down” the tree. CIS 550 Handout 7 Fall 2001 5

• XPath some nodes from a given Primary goal = to permit to access 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[parent/attribute: : booktitle = “XML”][2] • XPath also offers some shortcuts – no axis means child – // º /descendant-or-self: : node()/

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) context node 1 4 bbb aaa 5 2 3 ccc aaa 6 bbb aaa 7 ccc –. -- the context node – / -- the root node CIS 550 Handout 7 Fall 2001 7

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 CIS 550 Handout 7 Fall 2001 8

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”). – – CIS 550 Handout 7 Fall 2001 9

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/(employee|consultant) --as in regular path expressions -- is not allowed • However person/node()[boolean(employee|consultant)] 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 Na. N • 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 CIS 550 Handout 7 Fall 2001 10

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-or-self, followingsibling, namespace, parent, preceding-sibling, self – Some of these (self, parent) describe single nodes, others describe sequences of nodes. CIS 550 Handout 7 Fall 2001 11

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

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

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] ? CIS 550 Handout 7 Fall 2001 14

XQuery 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 CIS 550 Handout 7 Fall 2001 15

XQuery = XPath + “comprehension” syntax bind variables • XML -QL where in … use variables construct • Quilt CIS 550 Handout 7 bind variables for x in y in … use variables where return Fall 2001 16

Examples from XQuery List the titles of books published by Morgan Kaufmann in 1998. FOR $b IN document("bib. xml")//book WHERE $b/publisher = "Morgan Kaufmann" AND $b/year = "1998" RETURN $b/title XPath expressions in orange CIS 550 Handout 7 Fall 2001 17

Examples from XQuery (cont) List each publisher and the average price of its books. FOR $p IN distinct(document("bib. xml")//publisher) LET $a : = avg( document("bib. xml")//book[publisher = $p]/price) RETURN {$p/text()} {$a} LET binds a variable to a value. It does not cause an iteration. Does this create a (well-formed) XML document? CIS 550 Handout 7 Fall 2001 18

Examples from XQuery (cont) List the publishers who have published more than 100 books. { FOR $p IN distinct(document("bib. xml")//publisher) LET $b : = document("bib. xml")//book[publisher = $p] WHERE count($b) > 100 RETURN $p } What about efficiency? CIS 550 Handout 7 Fall 2001 19

Examples from XQuery (cont) Invert the structure of the input document so that each distinct author element contains a sequence of book-titles. { FOR $a IN distinct(document("bib. xml")//author) RETURN {$a/text()} { FOR $b IN document("bib. xml")//book[author = $a] RETURN $b/title } } CIS 550 Handout 7 Fall 2001 20

More Examples (Quilt)(from http: //db. cis. upenn. edu/Kweelt/use. Cases/R/Q 1. qlt ) Relational data -- two DTDs: ]> ]> CIS 550 Handout 7 Fall 2001 21

The data 1001 Red Bicycle U 01 1999 -01 -05 1999 -01 -20 40 1002 Motorcycle U 02 1999 -02 -11 1999 -03 -15 500 U 02 1001 35 99 -01 -07 U 04 1001 40 99 -01 -08 … … CIS 550 Handout 7 Fall 2001 22

(" src="https://present5.com/presentation/5c7d7d389056971295b7e721d7e0b054/image-23.jpg" alt="Query 1 FUNCTION date() { "1999 -02 -01" } simple function definitions (" /> Query 1 FUNCTION date() { "1999 -02 -01" } simple function definitions ( FOR $i IN document("items. xml")//item_tuple WHERE $i/start_date LEQ date() AND $i/end_date GEQ date() dates are formatted so AND contains($i/description, "Bicycle") that lexicographic RETURN ordering gives the right result $i/itemno , $i/description SORTBY (itemno) ) CIS 550 Handout 7 Fall 2001 23

1003" src="https://present5.com/presentation/5c7d7d389056971295b7e721d7e0b054/image-24.jpg" alt="Output from Q 1 1003" /> Output from Q 1 1003 Old Bicycle 1007 Racing Bicycle CIS 550 Handout 7 Fall 2001 24

Query Q 2 For all bicycles, list the item number, description, and highest bid (if any), ordered by item number. ( FOR $i IN document("items. xml")//item_tuple LET $b : = document("bids. xml")//bid_tuple[itemno = $i/itemno] WHERE contains($i/description, "Bicycle") RETURN $i/itemno , $i/description , IF ($b) THEN Num. Format("#####. ##", max(-1, $b/bid)) ELSE "" SORTBY (itemno) ) lots of coercion CIS 550 Handout 7 Fall 2001 25

Output from Q 2 1001 Red Bicycle 55 1003 Old Bicycle 20 1007 Racing Bicycle 225 1008 Broken Bicycle CIS 550 Handout 7 Fall 2001 26

Query Q 3 Find cases where a user with a rating worse (alphabetically greater than "C" ) offers an item with a reserve price of more than 1000. ( 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 Comparing sets with singletons AND $i/offered_by = $u/userid Same rules as in XPath? In this RETURN case the DTD gives uniqueness $u/name/text(), $u/rating/text(), $i/description/text(), $i/reserve_price ) CIS 550 Handout 7 Fall 2001 27