UNIT 1: BASIC CONCEPTS OF DATABASE MANAGEMENT SYSTEMS

Not satisfied with the navigational model of the Codasyl approach, notably the lack of a ―search‖ facility which was becoming increasingly useful, Edgar Codd, wrote a number of papers that outlined a new approach to database construction in 1970. He described a new system for storing and working with large databases. Instead of records being stored in some sort of linked list of free-form records as in Codasyl, Codd's idea was to use a ―table‖ of fixed-length records. A linked-list system would be very inefficient when storing ―sparse‖ databases where some of the data for any one record could be left empty. The relational model solved this by splitting the data into a series of normalised tables, with optional elements being moved out of the main table to where they would take up room only if needed.

Fig. 1.1:        The Relational Model

 In the relational model, related records are linked together with a ―key‖. For instance, a common use of a database system is to track information about users, their name, login information, various addresses and phone numbers. In the navigational approach all of these data would be placed in a single record, and unused items would simply not be placed in the database. In the relational approach, the data would be normalised into a user table, an address table and a phone number table (for instance). Records would be created in these optional tables only if the address or phone numbers were actually provided.

Linking the information back together is the key to this system. In the relational model, some bit of information was used as a ―key‖, uniquely defining a particular record. When information was being collected about a user, information stored in the optional (or related) tables would be found by searching for this key. For instance, if the login name of a user is unique, addresses and phone numbers for that user would be recorded with the login name as its key. This ―re-linking‖ of related data back into a single collection is something that traditional computer languages are not designed for.

Just as the navigational approach would require programs to loop in order to collect records, the relational approach would require loops to collect information about any one record. Codd‘s solution to the necessary looping was a set-oriented language, a suggestion that would later spawn the ubiquitous SQL. Using a branch of mathematics known as tuple calculus, he demonstrated that such a system could support all the operations of normal databases (inserting, updating etc.) as well as providing a simple system for finding and returning sets of data in a single operation.

IBM itself did one test implementation of the relational model, PRTV, and a production one, Business System 12, both now discontinued. All other DBMS implementations usually called relational are actually SQL DBMSs. In 1968, the University of Michigan began development of the Micro DBMS relational database management system. It was used to manage very large data sets by the US Department of Labor, the Environmental Protection Agency and researchers from University of Alberta, the University of Michigan and Wayne State University. It ran on mainframe computers using Michigan Terminal System. The system remained in production until 1996.