【正文】 ort and update the data. Hierarchical Model The first database management systems used a hierarchical modelthat isthey 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 ,the 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 monly found in business applications. As you have known, an organization char often describes a hierarchical relationship: 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. Hierarchical data are characterized by this onetomany 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 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 uses 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 containing information on employees, a record 揚(yáng)州大學(xué)計(jì)算機(jī)專業(yè) 畢業(yè)設(shè)計(jì) (外文翻譯 ) 3 would contain fields of data such as a person’s 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 Englishlike description that specifies the operation and the described record or bination of records. A query optimizer translates the description into a procedure to perform the database manipulation. Network Model The work model creates relationships among data through a linkedlist structure in which subordinate records can be linked to more than one parent record. This approach bines records with links, which are called pointers. The pointers are addresses that indicate the location of a record. With the work approach, a subordinate record can be linked to a key record and at the same time itself be a key record linked to other sets of subordinate records. The work mode historically has had a performance advantage over other database models. Today , such performance characteristics are only important in highvolume ,highspeed transaction processing such as automatic teller machine works or airline reservation system. Both hierarchical and work databases are application specific. If a new application is developed ,maintaining the consistency of databases in different applications can be very difficult. For example, suppose a new pension application is developed .The data are the same, but a new database must be created. Object Model The newest approach to database management uses an object model , in which records are represented by entities called objects that can both store data and provide methods or procedures to perform specific tasks. The query language used for the object model is the same objectoriented programming language used to develop the database application .This can create problems because there is no simple , uniform query language such as SQL . The object model is relatively new, and only a few examples of objectoriented database exist. It has attracted attention because developers who choose an objectoriented programming language want a database based on an objectoriented model. Distributed Database Similarly , a distributed database is one in which different parts of the database reside on physically separated puters . One goal of distributed databases is the access of information without regard to where the data might be stored. Keeping in mind that once the users and their data are sepa