2024年5月31日发(作者：)

Database Management

Database (sometimes spelled database) is also called an electronic database,

referring to any collections of data, or information, that is specially organized for

rapid search and retrieval by a computer. Databases are structured to facilitate the

storage, retrieval modification and deletion of data in conjunction with various

data-processing operations. Database can be stored on magnetic disk or tape,

optical disk, or some other secondary storage device.

A database consists of a file or a set of files. The information in the these files

may be broken down into records, each of which consists of one or more fields are

the basic units of data storage, and each field typically contains information

pertaining to one aspect or attribute of the entity described by the database.

Using keywords and various sorting commands, users can rapidly search,

rearrange, group, and select the fields in many records to retrieve or create reports

on particular aggregates of data.

Database records and files must be organized to allow retrieval of the

information. Early system were arranged sequentially (i.e., alphabetically,

numerically, or chronologically); the development of direct-access storage devices

made possible random access to data via indexes. Queries are the main way users

retrieve database information. Typically the user provides a string of characters,

and the computer searches the database for a corresponding sequence and

provides the source materials in which those characters appear. A user can request,

for example, all records in which the content of the field for a person’s last name

is the word Smith.

The many users of a large database must be able to manipulate the

information within it quickly at any given time. Moreover, large business and other

organizations tend to build up many independent files containing related and

even overlapping data, and their data, processing activities often require the

linking of data from several files. Several different types of database management

systems have been developed to support these requirements: flat, hierarchical,

network, relational, and object-oriented.

In flat databases, records are organized according to a simple list of entities;

many simple databases for personal computers are flat in structure. The records in

hierarchical databases are organized in a treelike structure, with each level of

records branching off into a set of smaller categories. Unlike hierarchical databases,

which provide single links between sets of records at different levels, network

databases create multiple linkages between sets by placing links, or pointers, to

one set of records in another; the speed and versatility of network databases have

led to their wide use in business. Relational databases are used where associations

among files or records cannot be expressed by links; a simple flat list becomes one

table, or “relation”, and multiple relations can be mathematically associated to

yield desired information. Object-oriented databases store and manipulate more

complex data structures, called “objects”, which are organized into hierarchical

classes that may inherit properties from classes higher in the chain; this database

structure is the most flexible and adaptable.