计算机外文翻译外文文献英文文献数据库系统

外文资料原文Database Systems1.Introduction to Database SystemToday, more than at any previous time, the success of an organization depends on its ability to acquire accurate and timely data about its operation, to manage this data effectively, and to use it to analyze and guide its activities. Phrases such as the information superhighway have become ubiquitous, and information processing is a rapidly growing multibillion dollar industry .The amount of information available to us is literally exploding, and the value of data as an organizational asset is being widely recognized. This paradox drives the need for increasingly powerful and flexible data management systems .A database is a collection of data , typically describing the activities of one or more related organizations . For example , a university database might contain information about the following .●Entities such as students , faculty , courses , and classrooms .●Relationships between entities , such as students’enrollment in courses , faculty teaching courses , and the use of rooms for courses .A database management system , or DBMS , is software designed to assist in maintaining and utilizing large collections of data , and the need for such systems , as well as their use , is growing rapidly . The alternative to using a DBMS is to use ad hoc approaches that do notcarry over from one application to another , for example , to store the data in files and write application-specific code to manage it .The area of database management systems is a microcosm of computer science in general . The issues addressed and the techniques used span a wide spectrum , including languages , object-orientation and other programming paradigms , compilation , operating systems1concurrent programming , data structures , algorithms ,theory , parallel and distributed systems , user interfaces , expert systems and artificial intelligence , statistical techniques , and dynamic programming .Database management continues to gain importance as more and more data is brought on-line, and made ever more accessible through computer networking. Today the field is being driven by exciting visions such as multimedia databases, interactive video, digital libraries, a host of scientific projects such as the human genome mappin g effort and NASA’s Earth Observation System project, and the desire of companies to consolidate their decision-making processes and mine their data repositories for useful information about their business . Commercially , database management systems represent one of the largest and most vigorous market segments . Thus the study of database systems couldprove to be richly rewarding in more ways than one .2.Database consistsA 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 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 systems 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 computersearches 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.In flat databases , records are organized according to a simplelist 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, orpointers, 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 struct ures, called “objects”, which are organized into hierarchical classes that may inherit properties from classes higher in the chain; this database structure is the mostflexible and adaptable.3.Structure of the Relational databaseThe relational model is the basis for any relational database management system (RDBMS).A relational model has three core components: a collection of objects or relations, operators that act on the objects or relations, and data integrity methods. In other words, it has a place to store the data, a way to create and retrieve the data, and a way to make sure that the data is logically consistent.A relational database uses relations, or two-dimensional tables, to store the information needed to support a business.3.1.Tables, Row, and ColumnsA table in a relational database, alternatively known as a relation, is a two-dimensional structure used to hold related information. A database consists of one or more related tables.Note: Don't confuse a relation with relationships. A relation is essentially a table, and a relationship is a way to correlate, join, or associate two tables.A row in a table is a collection or instance of one thing, such as one employee or one line item on an invoice. A column contains all the information of a single type, and the piece of data at the intersection of a row and a column, a field, is the smallest piece of informationthat can be retrieved with the database's query language. For example, a table with information about employees might have a column calledLAST_NAME that contains all of the employees' last names. Data is retrieved from a table by filtering on both the row and the column.3.2.Primary Keys, Data types, and Foreign KeysRelation: A two-dimensional structure used to hold related information, also known as a table.Row: A group of one or more data elements in a database table that describes a person, place, or thing.Column: The component of a database table that contains all of the data of the same name and type across all rows.Primary Key: A column (or columns) in a table that makes the row in the table distinguishable from every other row in the same table.Data types: numeric values, character or alphabetic values, and date values.A foreign key enforces the concept of referential integrity in a relational database.Foreign Key: A column (or columns) in a table that draws its valuesfrom a primary or unique key column in another table. A foreign key assists in ensuring the data integrity of a table. Referential Integrity A method employed by a relational database system that enforces one-to-many relationships between tables.3.3.Data ModelingIn this process, the developer conceptualizes and documents all the tables for the database. One of the common methods for modeling a database is called ERA, which stands for entities, relationships, and attributes. The database designer uses an application that can maintain entities, their attributes, and their relationships. In general, anentity corresponds to a table in the database, and the attributes of the entity correspond to columns of the table.Data Modeling: A process of defining the entities, attributes, and relationships between the entities in preparation for creating the physical database.The data-modeling process involves defining the entities, defining the relationships between those entities, and then defining theattributes for each of the entities. Once a cycle is complete, it is repeated as many times as necessary to ensure that the designer is capturing what is important enough to go into the database. Let's take a closer look at each step in the data-modeling process.3.4. Defining the EntitiesFirst, the designer identifies all of the entities within the scope of the database application.The entities are the persons, places, or things that are important to the organization and need to be tracked in the database. Entitieswill most likely translate neatly to database tables.。