Relational Data-Base Management Systems

Before describing the relational model of data, we will briefly discuss some trends in data-base management which give motivation to the development of the relational model. The first large-scale, machine-readable collections of data were stored on external media such as cards or tape. Beginning in the late fifties and early sixties, data banks were being stored on-line using direct-access devices such as disks. Generalized software packages such as BDAM and ISAM [T2] were developed to aid programmers in accessing the data. During the late sixties and early seventies, the idea of an integrated data-base management system was developed. This concept allowed several applications to share a common bank of data, maintained and protected by a central system. In an integrated data-base environment, the data-base management system (DBMS) provides each application program with its own view of the common data, implements various operators for retrieval and update of data, and resolves interference between concurrent users. The overall trend which is visible in data-base management today is the following: users are becoming increasingly oriented toward the information content of their data, and decreasingly concerned with its repwaenta.tion details. Increasingly, the user interface of a modern DBMS deals with abstract information rather than with the various bits, pointers, arrays, lists, etc., which may be used to represent information. Responsibility for choosing an appropriate representation for the information is being assumed by the system and is not exposed to the end user; indeed, the representation of a given fact may change over time without the user being aware of the change. The general term for this trend away from representation details is data independence. If we attempt to extrapolate the trend toward data independence, we observe that most current DBMS present the user with a view of records connected in some sort of structure, such as a network or hierarchy. In such a view, information may be represented in at least three ways: 1) by the contenls of records (e.g., Smith’s employee record has DEPTNO = 50.); 2) by the connections between records e.g., Smith’s employee record occurs in the hierarchy below the department record for Dept. 50.); and