The Semistructured-Data Model

Programming Languages for XML

Spring 2011

Instructor: Hassan Khosravi

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)

11.3

Semistructured Data representation

11.4

Relational Semistructured

Structure Tables Hierarchical tree,

graph

Schema Fixed in advance Flexible, self

describing

Queries Simple nice language Less so

Ordering None (has order by) Implied

Implementation Mature and native Add-on

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

11.6

Flexibility in Schema

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 DTD.

The DTD describes allowable tags and their nesting.

But still reasonably flexible – e.g. may allow optional or missing

fields

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

11.9

Well-Formed XML

Valid XML

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

QUERYING SEMISTRUCTURED

DATA

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

11.13

XPath

Think of XML as a tree

path expressions + conditions

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::

11.15

Xpath Demo

Example

11.16

XQuery

11.17

XQuery Demo

Example