1、 中文3300字Database Management Systems( 3th Edition ),Wiley ,2004, 5-12 A introduction to Database Management System Raghu Ramakrishnan A database (sometimes spelled data base) is also called an electronic database , referring to any collection of data, or information, that is specially organized
2、 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 .Databases can be stored on magnetic disk or tape, optical disk, or some other secondary storage de
3、vice. A database consists of a file or a set of files. The information in these files may be broken down into records, each of which consists of one or more fields. Fields are the basic units of data storage , and each field typically contains information pertaining to one aspect or attribute
4、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 aggregate of data. Complex data relationships and linkages may be found in all b
5、ut the simplest databases .The system software package that handles the difficult tasks associated with creating ,accessing, and maintaining database records is called a database management system(DBMS).The programs in a DBMS package establish an interface between the database itself and the users o
6、f the database. (These users may be applications programmers, managers and others with information needs, and various OS programs.) A DBMS can organize, process, and present selected data elements form the database. This capability enables decision makers to search, probe, and query database c
7、ontents in order to extract answers to nonrecurring and unplanned questions that arent available in regular reports. These questions might initially be vague and/or poorly defined ,but people can “browse” through the database until they have the needed information. In short, the DBMS will “man
8、age” the stored data items and assemble the needed items from the common database in response to the queries of those who arent programmers. A database management system (DBMS) is composed of three major parts:(1)a storage subsystem that stores and retrieves data in files；(2) a modeling and ma
9、nipulation subsystem that provides the means with which to organize the data and to add , delete, maintain, and update the data；(3)and an interface between the DBMS and its users. Several major trends are emerging that enhance the value and usefulness of database management systems； Managers: who re
10、quire more up-to-data information to make effective decision Customers: who demand increasingly sophisticated information services and more current information about the status of their orders, invoices, and accounts. Users: who find that they can develop custom applications with database systems in
11、 a fraction of the time it takes to use traditional programming languages. Organizations : that discover information has a strategic value； they utilize their database systems to gain an edge over their competitors. The Database Model A data model describes a way to structure and manipulate th
12、e data in a database. The structural part of the model specifies how data should be represented(such as tree, tables, and so on ).The manipulative part of the model specifies the operation with which to add, delete, display, maintain, print, search, select, sort and update the data. Hierarchical Mod
13、el The first database management systems used a hierarchical model-that is-they arranged records into a tree structure. Some records are root records and all others have unique parent records. The structure of the tree is designed to reflect the order in which the data will be used that is ,th
14、e record at the root of a tree will be accessed first, then records one level below the root ,and so on. The hierarchical model was developed because hierarchical relationships are commonly found in business applications. As you have known, an organization char often describes a hierarchical relatio
15、nship: top management is at the highest level, middle management at lower levels, and operational employees at the lowest levels. Note that within a strict hierarchy, each level of management may have many employees or levels of employees beneath it, but each employee has only one manager. Hierarchi
16、cal data are characterized by this one-to-many relationship among data. In the hierarchical approach, each relationship must be explicitly defined when the database is created. Each record in a hierarchical database can contain only one key field and only one relationship is allowed between any two
17、fields. This can create a problem because data do not always conform to such a strict hierarchy. Relational Model A major breakthrough in database research occurred in 1970 when E. F. Codd proposed a fundamentally different approach to database management called relational model ,which u
18、ses a table as its data structure. The relational database is the most widely used database structure. Data is organized into related tables. Each table is made up of rows called and columns called fields. Each record contains fields of data about some specific item. For example, in a table containi
19、ng information on employees, a record would contain fields of data such as a persons last name ,first name ,and street address. Structured query language(SQL)is a query language for manipulating data in a relational database .It is nonprocedural or declarative, in which the user need only specify an
20、 English-like description that specifies the operation and the described record or combination of records. A query optimizer translates the description into a procedure to perform the database manipulation. Network Model The network model creates relationships among data through a linked-list struct
21、ure in which subordinate records can be linked to more than one parent record. This approach combines records with links, which are called pointers. The pointers are addresses that indicate the location of a record. With the network approach, a subordinate record can be linked to a key record and at
22、 the same time itself be a key record linked to other sets of subordinate records. The network mode historically has had a performance advantage over other database models. Today , such performance characteristics are only important in high-volume ,high-speed transaction processing such as automatic teller machine networks or airline reservation system.
