Solution Sheet 5 XML Data Models and XQuery

Transcription

1 The Systems Group at ETH Zurich Big Data Fall Semester 2012 Prof. Dr. Donald Kossmann Prof. Dr. Nesime Tatbul Assistants: Martin Kaufmann Besmira Nushi Solution Sheet 5 XML Data Models and XQuery Exercise 1: XDM 1.1. An instance of the XDM is an ordered sequence of items. This way, the output of a query does not need to be a well formed XML document. Furthermore, XML is ordered, mixing up the order can have bad results. The XDM allows decomposing and reusing all expressions in XQuery Items are either atomic values or nodes, so that an output of a query does not need to be an XML fragment either. This can be helpful in many situations In the XQuery Data Model, there are sequences, atomic types and complex types, unlike in the Information Set, where there are only DTD attribute types. 1

2 Exercise 2: Infoset 2.1. The Information Set looks like the following. Note that we did not detail the Character Information Items. For convenience, we gave names to the Items so as to refer to them, but these names are not part of the Information Set. Document Information Item [children] Element Information Item metadata [document element] Element Information Item metadata [notations] empty [unparsed entities] empty [base URI ].../info.xml [character encoding scheme] UTF 8 [standalone] no value [version] 1.0 Element Information Item metadata [local name] metadata [children] Element Information Items title, publisher [attributes] empty [namespace attributes] Attribute Information Item namespacedc [in scope namespaces] Namespace Information Item namespacedcsystems, Namespace Information Item namespacexml [base URI].../info.xml [parent] Document Information Item Attribute Information Item namespacedc [namespace name] [local name] dc [prefix] xmlns [normalized value] [specified] true [attribute type] no value (no DTD!) [references] unknown [owner element] Element Information Item metadata Namespace Information Item namespacedcsystems Namespace Information Item namespacexml [prefix] xml [namespace name] 2

3 Element Information Item title [local name] title [children] Character Information Items, as many as characters in "Systems Group" [attributes] Attribute Information Items lang, year [namespace attributes] empty [in scope namespaces] Namespace Information Item namespacedcsystems, Namespace Information Item namespacexml [base URI].../info.xml [parent] Element Information Item metadata Attribute Information Item lang [namespace name] [local name] lang [prefix] xml [normalized value] "en" [specified] true [attribute type] no value (no DTD!) [references] unknown [owner element] Element Information Item title Attribute Information Item year [namespace name] empty [local name] year [prefix] empty [normalized value] "2008" [specified] true [attribute type] no value (no DTD!) [references] unknown [owner element] Element Information Item title Element Information Item publisher [local name] publisher [children] Character Information Items, as many as characters in "ETH Zurich" [attributes] empty [namespace attributes] empty [in scope namespaces] Namespace Information Item namespacedcsystems, Namespace Information Item namespacexml [base URI].../info.xml [parent] Element Information Item metadata 3

4 Exercise 3: Validation and Post-Schema-Validation Infoset (PSVI) 3.1. Unlike in the Information Set, there is type information in the PSVI A possible schema is the following: <?xml version="1.0" encoding="utf-8"?> <xs:schema xmlns:xs=" targetnamespace=" xmlns:dc=" elementformdefault="qualified"> <xs:import namespace=" schemalocation=" <xs:element name="metadata" type="dc:metadata-type"/> <xs:complextype name="metadata-type"> <xs:sequence> <xs:element name="title"> <xs:complextype> <xs:simplecontent> <xs:extension base="xs:string"> <xs:attribute ref="xml:lang"/> <xs:attribute name="year" type="xs:integer"/> </xs:extension> </xs:simplecontent> </xs:complextype> </xs:element> <xs:element name="publisher" type="xs:string"/> </xs:sequence> </xs:complextype> </xs:schema> 3.3. For metadata: [type definition type] complex [type definition namespace] [type definition anonymous] false [type definition name] metadata type For title: [type definition type] complex [type definition namespace] [type definition anonymous] true [type definition name] <generated> For publisher: [type definition type] simple [type definition namespace] [type definition anonymous] false [type definition name] string For xml:lang [type definition type] simple [type definition namespace] [type definition anonymous] true [type definition name] <generated> For year: [type definition type] simple 4

5 [type definition namespace] [type definition anonymous] false [type definition name] integer Exercise 4: Simple queries in XQuery 4.1. for $b in doc("bib.xml")//book order by xs:float($b/price) descending, $b/title ascending return $b/title 4.2. count(doc("bib.xml")//book[author = 'Abiteboul']) 4.3. for $a in distinct-values(doc("bib.xml")//author) return <res> <name>{$a}</name> <count> { count(doc("bib.xml")//book[author = $a]) } </count> </res> Exercise 5: Namespaces in XQuery a. To refer to the date subelement: /def:eth/det:date b. To refer to the date attribute which is in no namespace: /def:eth/@date To refer to the date attribute which is in a namespace: declare namespace dbisns =" /def:eth/@dbisns:date Note: try the following query to understand the behaviour of XQuery: declare namespace dbisns = " <result> <result0>{//eth}</result0> <result1>{//def:eth}</result1> <result2>{//def:eth/@date}</result2> <result3>{//def:eth/@def:date}</result3> 5

6 <result5>{//def:eth/date}</result5> <result6>{//def:eth/def:date}</result6> </result> It will output: <result> <result0/> <result1> <eth xmlns=" xmlns:xmldb=" date=" " xmldb:date=" "> <date> </date> <president number="1">empty</president> <Rektor>Name 2</Rektor> </eth> </result1> <result2 date=" "/> <result3/> <result4 xmlns:xmldb=" xmldb:date=" "/> <result5/> <result6> <date xmlns=" xmlns:xmldb=" > </date> </result6> </result> Exercise 6: Properties of XQuery Comparison Operators 6.1. $x := (1,2) (1,2) = 1 (1,2) = 2 = is not transitive, so one can not infer 1= $x := 1, $y := (0, 3), $y := 2 1 > (0,3) and (0,3) > 2 but 1 > 2 is false. > is not transitive either $x := () because () eq () returns () () = () returns false Exercise 7: Element constructors and node identities 7.1. let $a := <a/> let $b := <b>{$a}</b> return $b/a is $a Result: false 6

7 Reason: An element constructor always creates a copy which results in a new identity of the node let $a := <a/> let $b := $a return $b is $a Result: true Reason: same node, same identity 7