assign 01

Sunday, November 22, 2009


Refers to systems that are organized in the shape of a pyramid, with each row of objects linked to objects directly beneath it. Hierarchical systems pervade everyday life.

Hierarchical systems are as popular in computer systems as they are in other walks of life. The most obvious example of a hierarchical system in computers is a file system, in which directories have files and subdirectories beneath them. Such a file organization is, in fact, called a hierarchical file system . In addition to file systems, many data structures for storing information are hierarchical in form. Menu-driven programs are also hierarchical, because they contain a root menu at the top of the pyramid and submenus below it.


Its central idea is to describe a database as a collection of predicates over a finite set of predicate variables, describing constraints on the possible values and combinations of values. The content of the database at any given time is a finite (logical) model of the database, i.e. a set of relations, one per predicate variable, such that all predicates are satisfied. A request for information from the database (a database query) is also a predicate.

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