XML

1
XML

• Document Type Definitions
• XML Schema

2
Well-Formed and Valid XML

• Well-Formed XML allows you to invent your own
  tags.
• Valid XML conforms to a certain DTD.

3
Well-Formed XML

• Start the document with a declaration, surrounded
  by lt?xml ?gt .
• Normal declaration is
• lt?xml version 1.0 standalone yes ?gt
• standalone no DTD provided.
• Balance of document is a root tag surrounding
  nested tags.

4
Tags

• Tags are normally matched pairs, as ltFOOgt
  lt/FOOgt.
• Unmatched tags also allowed, as ltFOO/gt
• Tags may be nested arbitrarily.
• XML tags are case-sensitive.

5
Example Well-Formed XML

• lt?xml version 1.0 standalone yes ?gt
• ltBARSgt
• ltBARgtltNAMEgtJoes Barlt/NAMEgt
• ltBEERgtltNAMEgtBudlt/NAMEgt
• ltPRICEgt2.50lt/PRICEgtlt/BEERgt
• ltBEERgtltNAMEgtMillerlt/NAMEgt
• ltPRICEgt3.00lt/PRICEgtlt/BEERgt
• lt/BARgt
• ltBARgt
• lt/BARSgt

6
DTD Structure

• lt!DOCTYPE ltroot taggt
• lt!ELEMENT ltnamegt(ltcomponentsgt)gt
• . . . more elements . . .
• gt

7
DTD Elements

• The description of an element consists of its
  name (tag), and a parenthesized description of
  any nested tags.
• Includes order of subtags and their multiplicity.
• Leaves (text elements) have PCDATA (Parsed
  Character DATA ) in place of nested tags.

8
Example DTD

• lt!DOCTYPE BARS
• lt!ELEMENT BARS (BAR)gt
• lt!ELEMENT BAR (NAME, BEER)gt
• lt!ELEMENT NAME (PCDATA)gt
• lt!ELEMENT BEER (NAME, PRICE)gt
• lt!ELEMENT PRICE (PCDATA)gt
• gt

9
Element Descriptions

• Subtags must appear in order shown.
• A tag may be followed by a symbol to indicate its
  multiplicity.
• zero or more.
• one or more.
• ? zero or one.
• Symbol can connect alternative sequences of
  tags.

10
Example Element Description

• A name is an optional title (e.g., Prof.), a
  first name, and a last name, in that order, or it
  is an IP address
• lt!ELEMENT NAME (
• (TITLE?, FIRST, LAST) IPADDR
• )gt

11
Use of DTDs

• Set standalone no.
• Either
• Include the DTD as a preamble of the XML
  document, or
• Follow DOCTYPE and the ltroot taggt by SYSTEM and a
  path to the file where the DTD can be found.

12
Example (a)

• lt?xml version 1.0 standalone no ?gt
• lt!DOCTYPE BARS
• lt!ELEMENT BARS (BAR)gt
• lt!ELEMENT BAR (NAME, BEER)gt
• lt!ELEMENT NAME (PCDATA)gt
• lt!ELEMENT BEER (NAME, PRICE)gt
• lt!ELEMENT PRICE (PCDATA)gt
• gt
• ltBARSgt
• ltBARgtltNAMEgtJoes Barlt/NAMEgt
• ltBEERgtltNAMEgtBudlt/NAMEgt ltPRICEgt2.50lt/PRICEgtlt/BEER
  gt
• ltBEERgtltNAMEgtMillerlt/NAMEgt ltPRICEgt3.00lt/PRICEgtlt/B
  EERgt
• lt/BARgt
• ltBARgt
• lt/BARSgt

13
Example (b)

• Assume the BARS DTD is in file bar.dtd.
• lt?xml version 1.0 standalone no ?gt
• lt!DOCTYPE BARS SYSTEM bar.dtdgt
• ltBARSgt
• ltBARgtltNAMEgtJoes Barlt/NAMEgt
• ltBEERgtltNAMEgtBudlt/NAMEgt
• ltPRICEgt2.50lt/PRICEgtlt/BEERgt
• ltBEERgtltNAMEgtMillerlt/NAMEgt
• ltPRICEgt3.00lt/PRICEgtlt/BEERgt
• lt/BARgt
• ltBARgt
• lt/BARSgt

14
Attributes

• Opening tags in XML can have attributes.
• In a DTD,
• lt!ATTLIST E . . . gt
• declares attributes for element E, along with
  its datatype.

15
Example Attributes

• Bars can have an attribute kind, a character
  string describing the bar.
• lt!ELEMENT BAR (NAME BEER)gt
• lt!ATTLIST BAR kind CDATA IMPLIEDgt

16
Example Attribute Use

• In a document that allows BAR tags, we might see
• ltBAR kind sushigt
• ltNAMEgtHommaslt/NAMEgt
• ltBEERgtltNAMEgtSapporolt/NAMEgt
• ltPRICEgt5.00lt/PRICEgtlt/BEERgt
• ...
• lt/BARgt

17
IDs and IDREFs

• Attributes can be pointers from one object to
  another.
• Compare to HTMLs NAME foo and HREF foo.
• Allows the structure of an XML document to be a
  general graph, rather than just a tree.

18
Creating IDs

• Give an element E an attribute A of type ID.
• When using tag ltE gt in an XML document, give its
  attribute A a unique value.
• Example
• ltE A xyzgt

19
Creating IDREFs

• To allow elements of type F to refer to another
  element with an ID attribute, give F an
  attribute of type IDREF.
• Or, let the attribute have type IDREFS, so the F
  -element can refer to any number of other
  elements.

20
Example IDs and IDREFs

• A new BARS DTD includes both BAR and BEER
  subelements.
• BARS and BEERS have ID attributes name.
• BARS have SELLS subelements, consisting of a
  number (the price of one beer) and an IDREF
  theBeer leading to that beer.
• BEERS have attribute soldBy, which is an IDREFS
  leading to all the bars that sell it.

21
The DTD

• lt!DOCTYPE BARS
• lt!ELEMENT BARS (BAR, BEER)gt
• lt!ELEMENT BAR (SELLS)gt
• lt!ATTLIST BAR name ID REQUIREDgt
• lt!ELEMENT SELLS (PCDATA)gt
• lt!ATTLIST SELLS theBeer IDREF REQUIREDgt
• lt!ELEMENT BEER EMPTYgt
• lt!ATTLIST BEER name ID REQUIREDgt
• lt!ATTLIST BEER soldBy IDREFS IMPLIEDgt
• gt

22
Example A Document

• ltBARSgt
• ltBAR name JoesBargt
• ltSELLS theBeer Budgt2.50lt/SELLSgt
• ltSELLS theBeer Millergt3.00lt/SELLSgt
• lt/BARgt
• ltBEER name Bud soldBy JoesBar
• SuesBar /gt
• lt/BARSgt

23
Empty Elements

• We can do all the work of an element in its
  attributes.
• Like BEER in previous example.
• Another example SELLS elements could have
  attribute price rather than a value that is a
  price.

24
Example Empty Element

• In the DTD, declare
• lt!ELEMENT SELLS EMPTYgt
• lt!ATTLIST SELLS theBeer IDREF REQUIREDgt
• lt!ATTLIST SELLS price CDATA REQUIREDgt
• Example use
• ltSELLS theBeer Bud price 2.50 /gt

25
XML Schema

• A more powerful way to describe the structure of
  XML documents.
• XML-Schema declarations are themselves XML
  documents.
• They describe elements and the things doing the
  describing are also elements.

26
Structure of an XML-Schema Document

• lt? xml version ?gt
• ltxsschema xmlnsxs
• http//www.w3.org/2001/XMLschemagt
• . . .
• lt/xsschemagt

27
The xselement Element

• Has attributes
• name the tag-name of the element being defined.
• type the type of the element.
• Could be an XML-Schema type, e.g., xsstring.
• Or the name of a type defined in the document
  itself.

28
Example xselement

• ltxselement name NAME
• type xsstring /gt
• Describes elements such as
• ltNAMEgtJoes Barlt/NAMEgt

29
Complex Types

• To describe elements that consist of subelements,
  we use xscomplexType.
• Attribute name gives a name to the type.
• Typical subelement of a complex type is
  xssequence, which itself has a sequence of
  xselement subelements.
• Use minOccurs and maxOccurs attributes to control
  the number of occurrences of an xselement.

30
Example a Type for Beers

• ltxscomplexType name beerTypegt
• ltxssequencegt
• ltxselement name NAME
• type xsstring
• minOccurs 1 maxOccurs 1 /gt
• ltxselement name PRICE
• type xsfloat
• minOccurs 0 maxOccurs 1 /gt
• lt/xssequencegt
• lt/xscomplexTypegt

31
An Element of Type beerType

• ltxxxgt
• ltNAMEgtBudlt/NAMEgt
• ltPRICEgt2.50lt/PRICEgt
• lt/xxxgt

32
Example a Type for Bars

• ltxscomplexType name barTypegt
• ltxssequencegt
• ltxselement name NAME
• type xsstring
• minOccurs 1 maxOccurs 1 /gt
• ltxselement name BEER
• type beerType
• minOccurs 0 maxOccurs unbounded /gt
• lt/xssequencegt
• lt/xscomplexTypegt

33
xsattribute

• xsattribute elements can be used within a
  complex type to indicate attributes of elements
  of that type.
• attributes of xsattribute
• name and type as for xs.element.
• use required or optional.

34
Example xsattribute

• ltxscomplexType name beerTypegt
• ltxsattribute name name
• type xsstring
• use required /gt
• ltxsattribute name price
• type xsfloat
• use optional /gt
• lt/xscomplexTypegt

35
An Element of This New Type beerType

• ltxxx name Bud
• price 2.50 /gt

36
Restricted Simple Types

• xssimpleType can describe enumerations and
  range-restricted base types.
• name is an attribute
• xsrestriction is a subelement.

37
Restrictions

• Attribute base gives the simple type to be
  restricted, e.g., xsinteger.
• xsmin, maxInclusive, Exclusive are four
  attributes that can give a lower or upper bound
  on a numerical range.
• xsenumeration is a subelement with attribute
  value that allows enumerated types.

38
Example license Attribute for BAR

• ltxssimpleType name licensegt
• ltxsrestriction base xsstringgt
• ltxsenumeration value Full /gt
• ltxsenumeration value Beer only /gt
• ltxsenumeration value Sushi /gt
• lt/xsrestrictiongt
• lt/xssimpleTypegt

39
Example Prices in Range 1,5)

• ltxssimpleType name pricegt
• ltxsrestriction
• base xsfloat
• minInclusive 1.00
• maxExclusive 5.00 /gt
• lt/xssimpleTypegt

40
Keys in XML Schema

• An xselement can have an xskey subelement.
• Meaning within this element, all subelements
  reached by a certain selector path will have
  unique values for a certain combination of
  fields.
• Example within one BAR element, the name
  attribute of a BEER element is unique.

41
Example Key

• ltxselement name BAR gt
• . . .
• ltxskey name barKeygt
• ltxsselector xpath BEER /gt
• ltxsfield xpath _at_name /gt
• lt/xskeygt
• . . .
• lt/xselementgt

42
Foreign Keys

• An xskeyref subelement within an xselement says
  that within this element, certain values (defined
  by selector and field(s), as for keys) must
  appear as values of a certain key.

43
Example Foreign Key

• Suppose that we have declared that subelement
  NAME of BAR is a key for BARS.
• The name of the key is barKey.
• We wish to declare DRINKER elements that have
  FREQ subelements. An attribute bar of FREQ is a
  foreign key, referring to the NAME of a BAR.

44
Example Foreign Key in XML Schema

• ltxselement name DRINKERS
• . . .
• ltxskeyref name barRef
• refers barKey
• ltxsselector xpath
  DRINKER/FREQ /gt
• ltxsfield xpath _at_bar /gt
• lt/xskeyrefgt
• lt/xselementgt