Databases Reference
In-Depth Information
represented is inside the IS. In those cases, the IS is the only source of infor-
mation about the state, and the system becomes indispensable to its users.
To perform the informative function, the system needs an inference
capability, allowing it to infer the required information from the memory. In
the most frequent case, users pose a query, and the system answers it. Both
query and answer are in a language understood by the users and by the
system.
Queries may be extensional (the most frequent case) or intensional. An
extensional query asks information about the state of the domain (either the
current state or some previous one), and the system gives an extensional or
intensional answer. An extensional answer, which is the usual case, consists of
more or less elaborated information about the state of the domain. Examples
of simple extensional answers are:
Joan takes the Algebra course.
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Eighty students take the Programming course.
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Some extensional answers must be much more elaborated and may require
a statistical analysis, a simulation or the execution of a decisional model.
Examples of such answers are:
Ninety percent of customers that buy topics also buy CDs.
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No customer has bought more than 200 topics.
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Less frequently, the answer to an extensional query may be intensional.
An intensional answer characterizes the state of the domain, but it does not
describe the state explicitly [5]. For example, to the question Who earns
more than $100,000? the system might answer, The managers.
Intensional queries ask about the kinds of information the system
knows, rather than particular information [6], for example,
What do you know about students?
·
What is the maximum number of courses a student may take
simultaneously?
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What is a student?
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