Modelling Data on the Web note 4: XML Schema, XQuery, and robustness
Tree data models
- Data Structure formalisms: XML (including name spaces)
- Schema Language: XML Schema (XSD)
- Data Manipulation: XQuery
XML 有很多schema。可以通过 expressive power, easy of use/understanding, the complexity of validating a document w.r.t. a schema 来比较下。 其中 XML schema more expressive,have ns,gave datatyoe than DTDS
XML Schema another schema language for XML
an RNG schema in compact syntax is not a well-formed XML document
an XML Schema schema is a well-formed XML document
XML schema example
<?xml version="1.0"?>
<note xmlns= "http://www.w3schools.com"
xmlns:xs= "http://www.w3.org/2001/XMLSchema"
xmlns:xsi= "http://www.w3.org/2001/XMLSchema-instance">
<to>Tove</to>
<from>Jani</from>
<sentOn>2007-01-29</sentOn>
<body>
Have a nice weekend!
</body>
</note>
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace= "http://www.w3schools.com"
xmlns="http://www.w3schools.com" elementFormDefault="unqualified"> <xs:element name="note">
<xs:complexType>
<xs:sequence>
<xs:element name="to" type="xs:string"/>
<xs:element name="from" type="xs:string"/>
<xs:element name="sentOn" type="xs:date"/>
<xs:element name="body" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
XML Schema 注意点
XML Schema provides support for modularity & re-use through
- xs:import
- xs:include
- xs:redefine
An XSD schema typically has 2 namespaces:
- targetNamespace for elements which created by us.
- XML Schema namespace http://www.w3.org/2001/XMLSchema
XML Schema types
- xs:simpleType: attribute values and elements
- xs:complexType: elements with • element content or • mixed element contentor • text content and attributes
mixed content:
<xs:complexType name="PersonType" mixed="true">
<xs:sequence>
<xs:element name="Name" type="xs:string"/>
<xs:element name="DoB" type="xs:date"/>
</xs:sequence>
<xs:attribute name="friend" type="xs:boolean" default="true"/>
<xs:attribute name="phone" type="xs:string"/>
</xs:complexType>
XML Schema example2:
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns="http://www.w3schools.com"
elementFormDefault="unqualified">
<xs:element name="note">
<xs:complexType>
<xs:sequence>
<xs:element name="to" type="xs:string"/>
<xs:element name="from" type="xs:string"/>
<xs:element name="sentOn" type="xs:date"/>
<xs:element name="body" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
XML Schema: types
- xs:simpleType: attribute value, elements
- xs:complexType: elements with element content or mixed element content, text content and attri.
XML Schema: Atomic simple types
can be restricted using xs:restriction facets:
- enumeration:
<xs:simpleType name="bikeType">
<xs:restriction base="xs:string">
<xs:enumeration value="MTB"/>
<xs:enumeration value="Race"/>
<xs:enumeration value=“Children”/>
<xs:enumeration value=“Hybrid"/>
<xs:enumeration value=“Folding"/>
</xs:restriction>
</xs:simpleType>
- length
<xs:simpleType name="eightChar">
<xs:restriction base="xs:string">
<xs:length value="8"/>
</xs:restriction>
</xs:simpleType>
Schemas and Types； PSVI
- Post-schema-validation infoset: Internal Rep. adorned（带有） with schema information e.g., a tree adorned with default values & types （带有模式信息的内部代表）
- PSVI = DOM for XML document + XSD schema
Compare XML Schema & RelaxNG
In RNG, no restrictions & extension, no (non-atomic) types
XML Schema has restrictions on expressing constraints on content models
Self-Describing
XML is an external format for representing data
It’s Self-describing
- Destroyed by external validation,
- i.e., using application-specific schema for validation
XQuery
XPath, XQuery is based on node sequences
<?xml version="1.0" encoding="UTF-8"?>
<contactlist>
<person categ="friend" age="25">
<name>
<lastname>Doe</lastname>
<firstname>John</firstname>
</name>
<phone>0044 161 1234 5667</phone>
<address> 123 Main Street</address>
<city>Manchester</city>
</person>
for $b in
doc("people.xml")/contactlist/person
let $name as text() :=
if (xs:integer($b/@age) < xs:integer(16))
then ($b/name/firstname/text())
else ($b/name/lastname/text())
return $name
<sales>
{for $p in doc("contactlist-john-doe.xml")/contactlist/person
for $c in doc("cities.xml")/citylist/city
where $p/city = $c/name
return
(for $i in 1 to fn:count($c/club)
return concat("Dear ", $p/name/firstname,", do you like ", $c/club[$i], " ?"))}
</sales>
Quick Note on PSVI
Post Schema-Validation Infoset
- First approximation: DOM + Schema Information
- Remember node types in the DOM
PSVIs are known to be valid!
- Schema + Well formed XML => Validation=>PSVI
- Schema(create) + CSV => program => Populated database
error handling
Postel’s Law
- Be liberal in what you accept,
- and conservative in what you send.
But error handing 对于接受发送都 strict
Schemas
In SQL, schema before all
- CREATE TABLE or nothing happens
- Can’t INSERT INTO
- Can’t SELECT FROM
- So every SQL database has a schema
XML,never need a schema except the minimal schema of well-formed-ness 所以为什么有schema呢：
- To communicate
- To error check
- to guide tools