Background
The history of database research over the past 30 years is one of exceptional productivity that has led to the database system becoming arguably the most important development in the field of software engineering. The database is now the underlying framework of the information system and has fundamentally changed the way many organizations operate. In particular, the developments in this technology over the last few years have produced systems that are more powerful and more intuitive to use. This development has resulted in increasing availability of database systems for a wider variety of users. Unfortunately, the apparent simplicity of these systems has led to users creating databases and applications without the necessary knowledge to produce an effective and efficient system. And so the software crisis?or, as it is sometimes referred to, the software depression?continues.
The original stimulus for this book came from the authors?work in industry, providing consultancy on database design for new software systems or, as often as not, resolving inadequacies with existing systems. In addition, the authors?move to academia brought similar problems from different users students. The objectives of this book, therefore, are to provide a textbook that introduces the theory behind databases as clearly as possible and, in particular, to provide a methodology for database design that can be used by both technical and nontechnical readers.
The methodology presented in this book for relational Database Management Systems (DBMSs) the predominant system for business applications at present has been tried and tested over the years in both industrial and academic environments. It consists of three main phases: conceptual, logical, and physical database design. The first phase starts with the production of a conceptual data model that is independent of all physical considerations. This model is then refined in the second phase into a logical data model by removing constructs that cannot be represented in relational systems. In the third phase, the logical data model is translated into a physical design for the target DBMS. The physical design phase considers the storage structures and access methods required for efficient and secure access to the database on secondary storage.
The methodology in each phase is presented as a series of steps. For the inexperienced designer, it is expected that the steps will be followed in the order described, and guidelines are provided throughout to help with this process. For the experienced designer, the methodology can be less prescriptive, acting more as a framework or checklist. To help the reader use the methodology and understand the important issues, the methodology has been described using a realistic worked example, based on an integrated case study, DreamHome. In addition, three additional case studies are provided in Appendix B to allow readers to try out the methodology for themselves.
UML (Unified Modeling Language)
Increasingly, companies are standardizing the way in which they model data by selecting a particular approach to data modeling and using it throughout their database development projects. A popular high-level data model used in conceptual/ logical database design, and the one we use in this book, is based on the concepts of the Entity-Relationship (ER) model. Currently there is no standard notation for an ER model. Most books that cover database design for relational DBMSs tend to use one of two conventional notations:
● Chen notation, consisting of rectangles representing entities and diamonds representing relationships, with lines linking the rectangles and diamonds; or
● Crow Feet notation, again consisting of rectangles representing entities and lines between entities representing relationships, with a crow誷 foot at one end of a line representing a one-to-many relationship.
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