Download A Introduction to XQL

A Introduction of XQL
[ Part III of The XML seminar ]
Presenter: Xiaogeng Zhao
I. What is XQL?
XML Query Language ( XQL )
It is a notation for addressing and filtering
the elements and text of XML documents. XQL is a
natural extension to the XSL pattern syntax. It
provides a concise, understandable notation for
pointing to specific elements and for searching for
nodes with particular characteristics.
II. XQL Features
• XQL is a query language designed specifically
for XML.
• In XQL it is possible to combine information from
heterogeneous data sources in powerful ways.
• XQL specification does not indicate the output
format.
III. The XML Data Model
• A document is an ordered, labeled tree,
with nodes to represent the:
• document entity
• elements
• attributes
• processing instructions
• comments
The XML Data Model (Cont.)
The Tree Structure
(
XQL uses the tree structure for navigation )
• Hierarchy
• Sequence
• Position
III. SQL vs. XQL
• SQL
• XQL
1)
1)
The database is a set of one or
more XML documents.
The database is a set of tables.
2)
Queries are done in SQL, a
query language that uses the
structure of tables as a basic
model.
2)
Queries are done in XQL, a
query language that uses the
structure of XML documents as a
basic model.
3)
The FROM clause
determines the tables which
are examined by the query.
3)
A query is given a list of input
nodes from one or more
documents.
4)
The result of a query is a
table containing a set of rows;
this table may serve as the
basis for further queries.
4)
The result of a query is a list of
XML document nodes, which may
serve as the basis for further
queries.
IV. Basics of XQL
• Context
• start context
• Context operators
( /, //, [] )
Search Context
• XML Document
Basics of XQL
• A simple string is interpreted as an element
name.
• XQL query :
book
• Result :
return all <book> elements from the current
search context.
Basics of XQL
• The child operator ("/") indicates hierarchy.
• XQL query :
book/front/author
• Result :
returns <author> elements that are children of
<front> elements
Basics of XQL
• The root of a document may be indicated by a
leading "/" operator.
• XQL query :
/store/book/front/author
• Result :
Starting the navigation from the root: returns
<author> elements that are children of <front>
elements
Basics of XQL
• The content of an element or the value of an
attribute may be specified using the equals
operator ("=").
• XQL query :
book/front/author=‘Danny’
• Result :
return all authors with the name ”Danny" that
are children of the <front> element
Basics of XQL
• Attribute names begin with "@".They are treated
as children of the elements to which they belong.
• XQL query :
book/front/author/address/@type
• Result :
Finding values of the “type” attribute in
addresses of authors:
Basics of XQL
• The descendant operator ("//") indicates any
number of intervening levels. When the
descendant operator is found at the start of a
path, it means all nodes descended from the
document.
• XQL query :
//address
• Result :
This query will find any address in the document
Basics of XQL
• The filter operator ("[ ]") filters the set of nodes
to its left, based on the conditions inside the
brackets.
• XQL query :
book/front/author/address[@type='email']
• Result :
This query returns addresses; each of these
addresses must have an attribute called "type"
with the value "email"
Basics of XQL
• Multiple conditions may be combined using
Boolean operators.
• XQL query :
book/front/author=‘Tom'[@gender='male' and
@description =‘good guy']
• Result :
• Return the author element with the name of Tom
whose gender is male and is a good guy
Advance XQL
• Join
• Comprehension
• Methods…
Other XML Query Languages
• XML-QL
• Lorel
• YATL
XQL is A Query Language for ”Documents”
while XML-QL Lorel and YATL are
more ”Database” oriented
END of PART III