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Query languages
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Abstract
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The Query Languages session will provide delegates with an exploration
across the breadth of query standards and mechanisms, including both theory
and case studies.
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Contents
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Developing an XML query language
On 31st January 2000 W3C published a first draft of its XML Query
Requirements [XQL]. These identify the following
requirements of any query language to be created for the querying of XML-encoded
data sources:
3.2 General Requirements
3.2.1 Query Language Syntax
The XML Query Language MAY have more than one syntax binding. One query
language syntax MUST be convenient for humans to read and write. One query
language syntax MUST be expressed in XML in a way that reflects the underlying
structure of the query.
3.2.2 Declarativity
The XML Query Language MUST be declarative. Notably, it MUST not enforce
a particular evaluation strategy.
3.2.3 Protocol Independence
The XML Query Language MUST be defined independently of any protocols
with which it is used.
3.2.4 Error Conditions
The XML Query Language MUST define standard error conditions that can
occur during the execution of a query, such as processing errors within expressions,
unavailability of external functions to the query processor, or processing
errors generated by external functions.
3.2.5 Updates
Version 1.0 of the XML Query Language MUST not preclude the ability
to add update capabilities in future versions.
3.2.6 Defined for Finite Instances
The XML Query Language MUST be defined for finite instances of the data
model. It MAY be defined for infinite instances.
3.3 XML Query Data Model
3.3.1 Reliance on XML Information Set
The XML Query Data Model relies on information provided by XML Processors
and Schema Processors, and it MUST ensure that it does not require information
that is not made available by such processors. For XML constructs found in
XML 1.0 or the Namespaces Recommendation, the XML Query Data Model MUST show
how the equivalent XML Query Data Model constructs are are defined in terms
of items in the XML Information Set. The XML Query Data Model SHOULD represent
all information items, or provide justification for any information items
omitted. For information found in the XML Schema, such as datatypes, the Data
Model MUST coordinate with the XML Schema Working Group to ensure that schema
processors may be relied on to provide the information needed to construct
the Data Model.
3.3.2 Datatypes
The XML Query Data Model MUST represent both XML 1.0 character data
and the simple and complex types of the XML Schema specification.
3.3.3 Collections
The XML Query Data Model MUST represent collections of documents and
collections of simple and complex values. (Note that collections are not part
of the current XML Infoset.)
3.3.4 References
The XML Query Data Model MUST include support for references, including
both references within an XML document and references from one XML document
to another.
3.3.5 Schema Availability
Queries MUST be possible whether or not a schema is available (in this
document, the term "schema" may refer to either an XML Schema or a DTD). If
a schema is available, the data model MUST represent any items which they
define for their instances, such as default attributes, entity expansions,
or data types. These items will not be present if a schema is not present.
3.3.6 Namespace Awareness
The XML Query Language and XML Query Language Data Model MUST be namespace
aware.
3.4 XML Query Functionality
3.4.1 Supported Operations
The XML Query Language MUST support operations on all data types represented
by the XML Query Data Model (see datatypes, collections, references).
3.4.2 Text and Element Boundaries
Operations on text MUST be applicable to text that spans element boundaries.
3.4.3 Universal and Existential Quantifiers
Operations on collections MUST include support for universal and existential
quantifiers.
3.4.4 Hierarchy and Sequence
Queries MUST support operations on hierarchy and sequence of document
structures.
3.4.5 Combination
The XML Query Language MUST be able to combine related information from
different parts of a given document or from multiple documents.
3.4.6 Aggregation
The XML Query Language MUST be able to compute summary information from
a group of related document elements (this operation is sometimes called "aggregation.")
3.4.7 Composition of Operations
The XML Query Language MUST support expressions in which operations
can be composed, including the use of queries as operands.
3.4.8 NULL Values
The XML Query Language MUST include support for NULL values. Therefore,
all operators MUST take NULL values into account, including logical operators.
3.4.9 Structural Preservation
Queries MUST be able to preserve the relative hierarchy and sequence
of input document structures in query results.
3.4.10 Structural Transformation
Queries MUST be able to transform XML structures and MUST be able to
create new structures.
3.4.11 References
Queries MUST be able to traverse intra- and inter-document references.
3.4.12 Identity Preservation
Queries MUST be able to preserve the identity of items in the XML Query
Data Model.
3.4.13 Operations on Literal Data
Queries SHOULD be able to operate on XML Query Data Model instances
specified with the query. We refer to such data as "literal data" in this
document.
3.4.14 Operations on Names
Queries SHOULD be able to operate on names, such as element names, attribute
names, and processing instruction targets, and to operate on combinations
of names and data.
3.4.15 Operations on Schemas
Queries SHOULD provide access to the XML schema or DTD for a document,
if there is one. If the schema is represented as a DTD, a mapping to an appropriate
XML Schema representation MAY be required.
3.4.16 Extensibility
The XML Query Language SHOULD support the use of externally defined
functions on all datatypes of the XML Query Data Model. The interface to such
functions SHOULD be defined by the Query Language, and SHOULD distinguish
these functions from functions defined in the Query Language. The implementation
of externally defined functions is not part of the Query Language.
3.4.17 Environment Information
The XML Query Language MUST provide access to information derived from
the environment in which the query is executed, such as the current date,
time, or user.
3.4.18 Closure
Queries MUST be closed with respect to the XML Query Data Model. Both
the input to a query and the output of a query MUST be defined purely in terms
of the XML Query Data Model. Non-XML sources such as traditional databases
or objects may be queried if they are given an XML Query Data Model representation.
Similarly, query results are defined purely in terms of the XML Query Data
Model. In software systems these results may be instantiated in any convenient
representation such as DOM nodes, hyperlinks, XML text, or various data formats.
In this session the presentations will describe various techniques for
meeting some or all of these requirements
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