Types of database models

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File systems of varying degrees of sophistication satisfied the need for information storage and processing for several years. However, large enterprises tended to build many independent files containing related and even overlapping data, and data-processing activities frequently required the linking of data from several files. It was natural, then, to design data structures and database management systems that supported the automatic linkage of files. Three database models were developed to support the linkage of records of different types. These are: (1) the hierarchical model, in which record types are linked in a treelike structure (e.g., employee records might be grouped under a record describing the departments in which employees work); (2) the network model, in which arbitrary linkages of record types may be created (e.g., employee records might be linked on one hand to employees' departments and on the other hand to their supervisors--that is, other employees); and (3) the relational model, in which all data are represented in simple tabular form.

In the relational model, the description of a particular entity is provided by the set of its attribute values, stored as one row of the table, or relation. This linkage of attribute values to provide a meaningful description of a real-world entity or a relationship among such entities forms a mathematical -tuple; in database terminology, it is simply called a tuple. The relational approach also supports queries (requests for information) that involve several tables by providing automatic linkage across tables by means of a "join" operation that combines records with identical values of common attributes. Payroll data, for example, could be stored in one table and personnel benefits data in another; complete information on an employee could be obtained by joining the tables on the employee's identification number. To support any of these database structures, a large piece of software known as a database management system (DBMS) is required to handle the storage and retrieval of data (via the file management system, since the data are physically stored as files on magnetic disk) and to provide the user with commands to query and update the database. The relational approach is currently the most popular, as older hierarchical data management systems, such as IMS, the information management system produced by IBM, are being replaced by relational database management systems such as IBM's large mainframe system DB2 or the Oracle Corporation's DBMS, which runs on large servers. Relational DBMS software is also available for workstations and personal computers.

The need for more powerful and flexible data models to support nonbusiness applications (e.g., scientific or engineering applications) has led to extended relational data models in which table entries need not be simple values but can be programs, text, unstructured data in the form of binary large objects (BLOBs), or any other format the user requires. Another development has been the incorporation of the object concept that has become significant in programming languages. In object-oriented databases, all data are objects. Objects may be linked together by an "is-part-of" relationship to represent larger, composite objects. Data describing a truck, for instance, may be stored as a composite of a particular engine, chassis, drive train, and so forth. Classes of objects may form a hierarchy in which individual objects may inherit properties from objects farther up in the hierarchy. For example, objects of the class "motorized vehicle" all have an engine; members of subclasses such as "truck" or "airplane" will then also have an engine. Furthermore, engines are also data objects, and the engine attribute of a particular vehicle will be a link to a specific engine object. Multimedia databases, in which voice, music, and video are stored along with the traditional textual information, are becoming increasingly important and also are providing an impetus toward viewing data as objects, as are databases of pictorial images such as photographs or maps. The future of database technology is generally perceived to be a merging of the relational and object-oriented views.