XML, XML Schema, XPath and XQuery Query Languages CS561 Slides collated from several sources, including D. Suciu at Univ. of Washington

XML Data

CS561 - Spring XML W3C standard to complement HTML origins: structured text SGML motivation: –HTML describes presentation –XML describes content HTML e XML subset SGML

CS561 - Spring From HTML to XML HTML describes the presentation

CS561 - Spring HTML Bibliography Foundations of Databases Abiteboul, Hull, Vianu Addison Wesley, 1995 Data on the Web Abiteboul, Buneman, Suciu Morgan Kaufmann, 1999

CS561 - Spring XML Foundations… Abiteboul Hull Vianu Addison Wesley 1995 … XML describes the content

CS561 - Spring XML Terminology tags: book, title, author, … start tag:, end tag: elements: …, … elements are nested empty element: abbrv. an XML document: single root element well formed XML document: if it has matching tags

CS561 - Spring XML: Attributes Foundations of Databases Abiteboul … 1995 attributes are alternative ways to represent data

CS561 - Spring More XML: Oids and References Jane Mary John oids and references in XML are just syntax

CS561 - Spring So Far Differences between “xml data” versus “relational data” ? –Data model? –Typed? –Homogeneity? –Correctness? –Usage/Purpose ?

CS561 - Spring “XML Data Model” Numerous competing models: Document Object Model (DOM): –class hierarchy (node, element, attribute,…) –defines API to inspect/modify the document XML query data model (formal)

CS561 - Spring XML Namespaces name ::= [prefix:]localpart … 15 …. … 15 ….

CS561 - Spring … … XML Namespaces syntactic:, semantic: provide URL for “shared” schema defined here

CS561 - Spring So Far What are “namespaces” good for ? Are they typically available for relational databases?

Schemas for XML

CS561 - Spring DTD - Element Type Definitions

CS561 - Spring XML Schemas generalizes DTDs (SGML derivative) now, instead uses XML syntax two main documents: structure and data types XML Schema more powerful but more complex

CS561 - Spring XML Schema </xsd:complexType </xsd:complexType DTD:

CS561 - Spring So Far Differences between “xml schema” versus “relational schema” ? –Purpose ? Do we need it ? –Definition time? –Strictness of typing ? –Underlying model ?

CS561 - Spring Elements versus Types in XML Schema DTD:

CS561 - Spring Types: –Simple types (integers, strings,...) –Complex types (regular expressions, like in DTDs) Element-type-element alternation: –Root element has a complex type –Complex type is a regular expression of elements –Those elements have their complex types... –... –Leaves have simple types Elements versus Types in XML Schema

CS561 - Spring Local and Global Types in XML Schema Local type: [define locally the person’s type] Global type: [define here the type ttt] Global types: can be reused in other elements

CS561 - Spring Local v.s. Global Elements in XML Schema Local element:... Global element:... Global elements: like in DTDs

CS561 - Spring Regular Expressions in XML Schema Recall the element-type-element alternation: [regular expression on elements] Regular expressions: A B C..

CS561 - Spring Regular Expressions in XML Schema Regular expressions: A B C = A B C A B C = A | B | C A B C = (A B C).. = (...)*.. = (...)?

CS561 - Spring Regular Expressions in XML Schema Recall the element-type-element alternation: [regular expression on elements] Regular expressions: A B C = A B C A B C = A | B | C A B C = (A B C).. = (...)*.. = (...)?

CS561 - Spring Attributes in XML Schema Attributes are associated with the type, not the element Only to complex types; more trouble if we want to add attributes to simple types.

CS561 - Spring Derived Types by Extensions Corresponds to inheritance

Key Constraints in XML

CS561 - Spring Keys in XML Schema Lawnmower Baby Monitor Lapis Necklace Sturdy Shelves Lawnmower Baby Monitor Lapis Necklace Sturdy Shelves XML: XML Schema for Key :

CS561 - Spring Keys in XML Schema In general, syntax is : Notes: All XPath expressions “start” at the element currently being defined The fields must identify a single “node”.

CS561 - Spring Keys in XML Schema Unique = guarantees uniqueness Key = guarantees uniqueness and existence All XPath expressions are “restricted”: –/a/b | /a/c OK for selector –//a/b/*/c OK for field Note: better than DTD’s ID mechanism

CS561 - Spring Examples of Keys in XML Schema Examples Note: Must have single firstname, Single surname

CS561 - Spring Foreign Keys in XML Schema Example

CS561 - Spring So Far Differences between “keys/foreign-keys”in xml versus relational model? –Purpose ? –Underlying model ?

XPath “The Basic Building Block”

CS561 - Spring XPath Goal = Permit access some nodes from document XPath main construct : Axis navigation Navigation step : axis + node-test + predicates Examples –descendant::node() –child::author –attribute::booktitle =“XML” XPath path consists of one or more navigation steps, separated by “/” Navigation step : axis + node-test + predicates Examples –/descendant::node()/child::author –/descendant::node()/child::author[parent/attribute::booktitle =“XML”][2]

CS561 - Spring XPath Goal = Permit access some nodes from document XPath main construct : Axis navigation Navigation step : axis + node-test + predicates Examples –descendant::node() –child::author –attribute::booktitle =“XML”

CS561 - Spring XPath XPath path consists of one or more navigation steps, separated by “/” Navigation step : axis + node-test + predicates Examples –/descendant::node() /child::author –/descendant::node() /child::author [parent /attribute::booktitle =“XML”][2] XPath offers shortcuts : –no axis means child –//  / descendant-or-self::node()/

CS561 - Spring XPath- Child Axis Navigation author is shorthand for child::author. Examples: –aaa -- all the children nodes labeled aaa –aaa/bbb -- all the bbb grandchildren of aaa children –*/ bbb all the bbb grandchildren of any child Notes: –. -- the context node –/ -- the root node aaa bbb cccaaa bbb ccc context node

CS561 - Spring XPath- Child Axis Navigation author is shorthand for child::author. Examples: –aaa -- all the children 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) Notes: –. -- the context node –/ -- the root node aaa bbb cccaaa bbb ccc context node

CS561 - Spring XPath- Child Axis Navigation –/ doc -- all doc children of the root –./ aaa -- all aaa children of the context node (equivalent to aaa ) –text() -- all text children of context node –node() -- all 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 –//text() -- all the text nodes in the document

CS561 - Spring Predicates –[2] -- the second child node of the context node –chapter[5] -- the fifth chapter child of 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” (string-value is concatenation of all 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 ”

CS561 - Spring Axis navigation So far, our expressions have moved us down by moving to children nodes. Exceptions are : –. stay where you are –/ go to the root –// all descendants of the root –.// all descendants of the context node

CS561 - Spring Axis navigation 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 describe single nodes: – self, parent Some describe sequences of nodes: –All others

CS561 - Spring XPath Navigation Axes ancestor descendant followingpreceding following-siblingpreceding-sibling child attribute namespace self

CS561 - Spring XPath Abbreviated Syntax ///descendant-or-self::node().self::node().//descendant-or-self::node..parent::node() /(document root)

CS561 - Spring XPath Widely adopted -- in XML-Schema and in many query languages. About as expressive as regular path expressions

CS561 - Spring So Far Differences between SQL and XPATH? What are similar query capabilities? What features does SQL have, but not XPATH? What features does XPATH support, but not SQL? Is XPath a full-fledged query language?

Query Languages - XQuery

CS561 - Spring Summary of XQuery FLWR expressions FOR and LET expressions Collections and sorting Resources XQuery: A Query Language for XML XQuery: A Query Language for XML Chamberlin, Florescu, et al. W3C recommendation:

CS561 - Spring XQuery Designed based on Quilt (which is based on XML-QL) XML Query data model (ordered)

CS561 - Spring FLWR (“Flower”) Expressions FOR... LET... FOR... LET... WHERE... RETURN...

CS561 - Spring XQuery Find the titles of all books 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 How does result look like?

CS561 - Spring 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

CS561 - Spring XQuery Example FOR $a IN ( document("bib.xml") /bib/book[publisher=“Morgan Kaufmann”]/author) RETURN $a, FOR $t IN /bib/book[author=$a]/title RETURN $t FOR $a IN ( document("bib.xml") /bib/book[publisher=“Morgan Kaufmann”]/author) RETURN $a, FOR $t IN /bib/book[author=$a]/title RETURN $t

CS561 - Spring XQuery Example FOR $a IN ( document("bib.xml") /bib/book[publisher=“Morgan Kaufmann”]/author) RETURN $a, FOR $t IN /bib/book[author=$a]/title RETURN $t FOR $a IN ( document("bib.xml") /bib/book[publisher=“Morgan Kaufmann”]/author) RETURN $a, FOR $t IN /bib/book[author=$a]/title RETURN $t For each author of a book by Morgan Kaufmann, list all books she published: What is query result ?

CS561 - Spring XQuery Result: Jones abc def Jones abc def Smith ghi

CS561 - Spring XQuery Example: Duplicates 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 distinct = a function that eliminates duplicates

CS561 - Spring Example XQuery Result Result: Jones abc def Smith ghi

CS561 - Spring XQuery FOR $x in expr – binds $x to each element in the list expr –Useful for iteration over some input list LET $x = expr – binds $x to the entire list expr –Useful for common subexpressions and for grouping and aggregations

CS561 - Spring XQuery with LET Clause count = a (aggregate) function that returns number of elements 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

CS561 - Spring XQuery Find books whose price is larger than average: LET $a = avg( document("bib.xml") FOR $b in document("bib.xml") /bib/book WHERE > $a RETURN $b LET $a = avg( document("bib.xml") FOR $b in document("bib.xml") /bib/book WHERE > $a RETURN $b

CS561 - Spring FOR versus LET FOR Binds node variables  iteration LET Binds collection variables  one value

CS561 - Spring 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 := document("bib.xml") /bib/book RETURN $x LET $x := document("bib.xml") /bib/book RETURN $x Returns:...

CS561 - Spring 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:...

CS561 - Spring XQuery Summary FOR-LET-WHERE-RETURN = FLWR FOR/LET Clauses WHERE Clause RETURN Clause List of tuples Instances of XQuery data model

CS561 - Spring XQuery Some more query features

CS561 - Spring 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, 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, SORTBY (price DESCENDING) SORTBY (name)

CS561 - Spring Sorting in XQuery Sorting arguments: refer to name space of RETURN clause, not of FOR clause TIP: To sort on an element you don’t want to display, first return it, then remove it with an additional query.

CS561 - Spring 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)

CS561 - Spring 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

CS561 - Spring 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

CS561 - Spring So Far Similarities between SQL and XQuery? Differences between SQL and XQuery?

XML, XML Data Model XML Schema, XPath XQuery