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Database and Data Warehouses
What is a Database?
What is a database? A database is a collection of related data items. It is generally stored on
secondary storage devices that allow rapid direct access to individual data items. Redundancy is
minimized; where possible, only a single copy if a data item exists. The database may be used by many
different application systems at once, eliminating the need for separate systems to maintain the data for
each application .When a user program inquires if a particular item is in the database, a database
management system (DBMS) does the actual searching. The user does not need to be familiar with the
format in which the data is stored or the actual physical location of the data.
The DBMS cerates the database, keeps it up-to-date, and provides reday access to authorized user.
Database management systems also provide extensive security measures to prevent unauthorized
access. They make it convenient for expressing relationship between related data items and facilitate
the design of user application systems. They provide backup and recovery capabilities to prevent
against loss or destruction of vital information. They ensure database integrity, that is, what is supposed
to be in the database is there and what is not supposed to be .isn’t. a person called the database
administrator determines who may access the database, modify it, add new relationships ,and the like.
This is a very important lifeline. The real question in database management systems is how to organize
information to provide rapid answers to the kinds of questions users are likely to ask. The same data
organized differently can yield drama-tidally different access speeds. There common ways are used to
structure a database to indicate the relationships among the data items; these are the relational database,
the hierarchical data-base, and the network database.
Hierarchical, Network, and Relational Databases (three principal logical database models)
The Hierarchical Model In a hierarchical database, data records are arranged in a strict parent child
relationship. Each parent record may have many children, but each child record has exactly one parent.
Figure 3-1 shows a simple hierarchical database, indicating the relationship between a customer and
the orders it has placed with a company. Searching a hierarchical database is rapid and convenient as
long as it is searched from the top down. IBM’s IMS (Information Management System) is the most
widely used hierarchical DBMS. Hierarchical DBMS are best suited for problems that require a limited
number of structure answers that can be specified in advance. Once data relationships have been
specified, they cannot easily be changed without a major programming effort. Thus, the hierarchical
model cannot respond flexibly to changing requests for information.
The Network Model The network database model is best at representing many-to-many relationship
among data. In other words, a ”child” can have more than one “parent”, as Figure3-2 illustrates.
Computer Associates ’IDMS is a network DBMS for computer mainframes.
Network DBMS are more flexible than hierarchical DBMS, but access paths must still be
specified in advance. There are practical limitations to the number of links, or relationships ,that can be
established among records. If they are too numerous, the software will not work efficiently. Neither
network no hierarchical database management models can easily create new relationships among data
elements or new patterns of access without major programming efforts.
The Relational Model In the early 1970s the relational database approach was developed to provide
a much more user-friendly organization. Instead of using complex structures of pointers the relational
database stores information simply as tables called relations. These tables are sometimes called flat
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资源评论
- 骷髅吹牛2014-05-19还可以 不过有点过时了~~~
- wkaoo72013-11-28翻译的很一般,原文也比较旧,还要5分。。。有点说不过去吧···
- tutushentong2012-07-04还可以 不过有点老了~~~
- holyromance22013-11-18非常好,对我有很多的帮助。
ZKFcoffee
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