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【正文】 base is created. Each record in a hierarchical database can contain only onekey field and only one relationship is allowed between any two fields. This can createa problem because data do not always conform to such a strict Model A major breakthrough in database research occurred in 1970 when E. F. Coddproposed a fundamentally different approach to database management calledrelational model which uses a table as its data structure. The relational database is the most widely used database structure. Data isanized into related tables. Each table is made up of rows called and columns calledfields. Each record contains fields of data about some specific item. For example in atable containing information on employees a record would contain fields of data suchas a person’s last name first name and street address. Structured query languageSQLis a query language for manipulating data in arelational database .It is nonprocedural or declarative in which the user need onlyspecify an Englishlike description that specifies the operation and the describedrecord or bination of records. A query optimizer translates the description into aprocedure to perform the database Model The work model creates relationships among data through a linkedliststructure in which subordinate records can be linked to more than one parent approach bines records with links which are called pointers. The pointersare addresses that indicate the location of a record. With the work approach asubordinate record can be linked to a key record and at the same time itself be a keyrecord linked to other sets of subordinate records. The work mode historically hashad a performance advantage over other database models. Today such performancecharacteristics are only important in highvolume highspeed transaction processingsuch as automatic teller machine works or airline reservation system. Both hierarchical and work databases are application specific. If a newapplication is developed maintaining the consistency of databases in differentapplications can be very difficult. For example suppose a new pension application isdeveloped .The data are the same but a new database must be Model The newest approach to database management uses an object model in whichrecords are represented by entities called objects that can both store data and providemethods or procedures to perform specific tasks. The query language used for the object model is the same objectorientedprogramming language used to develop the database application .This can createproblems because there is no simple uniform query language such as SQL . Theobject model is relatively new and only a few examples of objectoriented databasee
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