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Spring 2011 Instructor: Hassan Khosravi

The Semistructure d -Data Model Programming Languages for XML. Spring 2011 Instructor: Hassan Khosravi. Semistructured Data. Another data model, based on trees. Self-describing : The data implicitly carries information about what its schema is.

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Spring 2011 Instructor: Hassan Khosravi

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

  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. Semistructured Data representation

  4. 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

  5. Flexibility in Schema

  6. 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

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

  8. Well-Formed XML Valid XML

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

  10. Querying semistructured data

  11. 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

  12. XPath • Think of XML as a tree • path expressions + conditions

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

  14. Xpath Demo • Example

  15. XQuery

  16. XQuery Demo • Example

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