数据库和数据仓库------外文翻译.doc

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 Hierarchica