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Database Modeling & Design Fourth Edition- P28

Database Modeling & Design Fourth Edition- P28: Database technology has evolved rapidly in the three decades since the rise and eventual dominance of relational database systems. While many specialized database systems (object-oriented, spatial, multimedia, etc.) have found substantial user communities in the science and engineering fields, relational systems remain the dominant database technology for business enterprises. | 122 CHAPTER 6 Normalization Table 6.4 Candidate Tables and FDs from ER Diagram Transformation continued technician none skill skill_type - skill_descrip project location project_name - start_date end_date head_id loc_name - loc_county loc_state zip prof_assoc desktop assoc_name - assoc_addr phone_no start_date desktop_no - computer_type serial_no desktop_no - emp_no assigned_to skill_used emp_id loc_name - project_name none 6.4 Determining the Minimum Set of 3NF Tables A minimum set of 3NF tables can be obtained from a given set of FDs by using the well-known synthesis algorithm developed by Bernstein 1976 . This process is particularly useful when you are confronted with a list of hundreds or thousands of FDs that describe the semantics of a database. In practice the ER modeling process automatically decomposes this problem into smaller subproblems the attributes and FDs of interest are restricted to those attributes within an entity and its equivalent table and any foreign keys that might be imposed upon that table. Thus the database designer will rarely have to deal with more than ten or twenty attributes at a time and in fact most entities are initially defined in 3NF already. For those tables that are not yet in 3NF only minor adjustments will be needed in most cases. In the following text we briefly describe the synthesis algorithm for those situations where the ER model is not useful for the decomposition. In order to apply the algorithm we make use of the well-known Armstrong axioms which define the basic relationships among FDs. Inference rules Armstrong axioms Reflexivity If Y is a subset of the attributes of X then X - Y i.e. if X is ABCD and Y is ABC then X - Y. Trivially X - X Augmentation If X - Y and Z is a subset of table R i.e. Z is any attribute in R then XZ - YZ. 6.4 Determining the Minimum Set of 3NF Tables 123 Transitivity If X- Y and Y- Z then X- Z. Pseudotransitivity If X- Y and YW- Z then XW- Z. Transitivity is a special case of .