Chapter 7 - XML Data Modelling

Question 1

What are the goals and main characteristics of XML?

Answer 1

• original intent: document markup language (tags = extra information)
• separate document content from structure: describe documents for interchange
• meta-language: language for defining other languages
• markup = metadata at specific parts (instance-level): self-describing documents
  
  • > schema (optional) = “global metadata (vocabulary, strcture, allowed content …)

Question 2

Which application scenarios influence the development of XML?

Answer 2

• Document processing: use document in various, evolving systems
  
  • > structure, content, layout (grammar: markup vocabulary for mixed content)
• Databases and Data Exchange: data independence
  
  • > structured, typed data (schema, integrity constraints
• Semi-structured data and information integration: integrate autonomous data sources
  
  • > dynamic, partially known schemas (data analysis after processing)

Question 3

What is the main characteristic and structure of XML documents?

Answer 3

• > Unicode text files: markup: tags, references (&), comments, PI’s + data: text (character data) (att. sysntax: CDATA)
• > Well-formed: optional XML decl./ schema, single root, nesting rules, etc..

Question 4

(Additional Information on Elements, Attributes and Namespaces)

Answer 4

• Element content is ordered
• Mix of text and elemtns allowed but not recommended
• attribute values = string (no further structuring)
• attributes are unordered and unique within element
• attributes vs subelements:
  
  • > documents: attr is part of markup (interpretation), elements are basic context
  • > data representation: difference is unclear
• namespaces specify how to construct universally unique names
  
  • > collection of names identified by URI, declared via XMLNS: attribute (IMPORT ~)
• Just xmlns=”URI”=> declared NS

Question 5

What are the goals of a schema definition in XML?

Answer 5

• > Standardized data exchange (structured data)

- > Restrictions on structure and data types: validation of documents

Question 6

Overview DTD

Answer 6

• Defines elements, their mandatory/optional attributes and subelements (no. of occur)
• no data types: all values as strings
• <!DOCTYPE …> clause in XML doc
• Syntax <!ELEMENT elem-name (subelement specificatoin)> name list, #PCDATA, regex
  <!ATTLIST elem-name attr-spec> type: CDATA, ID, NMTOKEN, enumeration + #REQUIRED, #IMPLIED, def. value

untyped:

• ID type: distinct value (object identifier)
• IDREF type: ID value of elem. in same document
• IDREFS type: set of ID values (IDREF)

Question 7

Overview XML schema.

Answer 7

(class of documents -> instance documents conforms to it)

• closer to general understanding of a database schema. Add. to DTD, supports:
  
  • > typing/constraints of values, typed references, UDTs, XML syntax, namespaces, list types, inheritance, unique, foreign key… -> more complicated than DTD

Question 8

Overview: types in XML schema

Answer 8

• > simple: no further structure (child elem/attrs) -> plain values, ran be derived by restriction
  vs. complex: may carry attributes and have element content (or empty)
• > primitive: buit-in, simple types which are not derived
  vs. derived: defined in terms of other types (restriction, list, union, extension)
• > built-in vs. user-derived

Question 9

Overview: complex types

Answer 9

• simple content: add attributes to simple content of elements (extension)
• sequence (subelements in specified order), choice (one of subelems), all (unordered)
  
  • > subelements with number of occurrences (min occurs, max occurs)
• derivation: restriction/extension

Question 10

What are the alternatives for deriving types in XML schema?

Answer 10

• Restriction: same content of base type
  
  • > for simple content: add facets (bounds, lengths, enumeration, pattern)
  • > complex content: limiting occurrences (larger min/ smaller max)
    • specifying default or fixed attribute value
    • remove optional component
    • replace types with derived (simple types
• Extension: add attributes and elements to content inherited from base type
• • must be surrounded by or

Question 11

How does XML Schema support substitutability of derived types?

Answer 11

• document leve: element of derived type can occur in the place of base type
  
  • > requires attribute xsi: type to specify the derived type
  • > can be forbidden with block attribute (or final)
• Element level: specify which element names may substitute a head type
  
  • > must be of the same type or derived
  • > substitution groups!

Question 12

What are the namespaces involved in XML Schema and their associations?

Answer 12

• Schema vocabulary (schema of XML Schema): fixed URI, usually xsd prefix
• Target Namespace defines NS to which the names defined in the schema belong
• Doc using schema: defines xmlns as target Namespace (schemaLocation to get file)

Question 13

How does XML Schema support unique constraints?

Answer 13

xsd: unique element: selector + field
- > XPath expressions to define scope and values that must be unique within an element

xsd:key = unique + nillable = false

Question 14

How can XML schema map ER models with 1:n and n:m relationships?

Answer 14

• > entities = XML elements (xsd:key simulates ER keys)
• > 1:n relationships: nesting N subelements inside (sequence)
  • > by local element definition (element name=””) or global (elment ref=””)
  • > requires xsd:key/xsd:keyref for uniqueness + referential integrity
    • > nesting alone insufficient
• > n:m relationships: key+keyref in helper element (similar to relational table)
  • > flat modelling with pointers

Question 15

What are the advantages of key/key ref in XML schema over ID/IDREF in DTDs?

Answer 15

• based on equality of data types
• allows composite keys
• scope restriction (implies type)