1. The Semistructured-Data Model Programming Languages for XML Spring 2011 Instructor: Hassan Khosravi

2. 11.2 Semistructured Data Another data model, based on trees. Self-describing: The data implicitly carries information about what its schema is. May only carry the names of attributes (so possibly untyped), and has a lower degree of organization than the data in a relational database. May have no associated schema (i.e. may be schema-less) Motivation: flexible representation of data. sharing of documents among systems and databases.  Information integration –E.g. want to “merge” or query two databases.  Data exchange –E.g. two enterprises may want to exchange data (such as buyers and sellers)

3. 11.3 Semistructured Data representation

4. 11.4 RelationalSemistructured StructureTablesHierarchical tree, graph SchemaFixed in advanceFlexible, self describing QueriesSimple nice languageLess so OrderingNone (has order by)Implied ImplementationMature and nativeAdd-on

5. 11.5 Comparison with Relational Data Inefficient: tags, which in effect represent schema information, are repeated Access: data is structured hierarchically. Better than relational tuples as a data-exchange format Unlike relational tuples, semistructured data is self-documenting due to presence of tags Flexible, non-rigid format: tags can be added Allows nested structures Wide acceptance, not only in database systems, but also in browsers, tools, and applications

6. 11.6 Flexibility in Schema

7. 11.7 XML XML : Extensible Markup Language A standard adopted in 1998 While HTML uses tags for formatting (e.g., “italic”), XML uses tags for semantics (e.g., indicating “this is an address” or “this is a title”). Key idea: create tag sets for a domain (e.g., genomics), and translate all data into properly tagged XML documents. There are two different modes of use of XML: Well-Formed XML allows you to invent your own tags.  No predefined schema Valid XML conforms to a certain Document Type Descriptor DTD.  The DTD describes allowable tags and their nesting.  But still reasonably flexible – e.g. may allow optional or missing fields

8. 11.8 Well-Formed XML Begins with a declaration that it is XML It has a root element that is the entire body of the text

9. 11.9 Well-Formed XML Valid XML

10. 11.10 Valid XML Document Type Descriptor (DTD) Grammar-like language for specifying elements, attributes, nesting, ordering, #occurrences Special attribute types ID and IDREF Example

11. 11.11 QUERYING SEMISTRUCTURED DATA

12. 11.12 Querying XML Not nearly as mature as Querying relational Newer No underlying theory as in relational models Sequence of development Xpath – path expressions + conditions Xquery – Xpath + full featured query language

13. 11.13 XPath Think of XML as a tree path expressions + conditions

14. 11.14 Xpath Syntax /  root element name of element “book” Use name as * to match everything @ISBN //  matches all descendant conditions [@price < 50] [N]  nth child author [2] Axes (to navigate around tree 13) Parent:: Following-sibling:: Descendants:: Self::

15. 11.15 Xpath Demo Example

16. 11.16 XQuery

17. 11.17 XQuery Demo Example