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Database Modeling & Design Fourth Edition- P26: 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. | 112 CHAPTER 6 Normalization Consider the disadvantages of 1NF in table report. Report_no editor and dept_no are duplicated for each author of the report. Therefore if the editor of the report changes for example several rows must be updated. This is known as the update anomaly and it represents a potential degradation of performance due to the redundant updating. If a new editor is to be added to the table it can only be done if the new editor is editing a report both the report number and editor number must be known to add a row to the table because you cannot have a primary key with a null value in most relational databases. This is known as the insert anomaly. Finally if a report is withdrawn all rows associated with that report must be deleted. This has the side effect of deleting the information that associates an author_id with author_name and author_addr. Deletion side effects of this nature are known as delete anomalies. They represent a potential loss of integrity because the only way the data can be restored is to find the data somewhere outside the database and insert it back into the database. All three of these anomalies represent problems to database designers but the delete anomaly is by far the most serious because you might lose data that cannot be recovered. These disadvantages can be overcome by transforming the 1NF table into two or more 2NF tables by using the projection operator on the subset of the attributes of the 1NF table. In this example we project report over report_no editor dept_no dept_name and dept_addr to form reportl and project report over author_id author_name and author_addr to form report2 and finally project report over report_no and author_id to form report3. The projection of report into three smaller tables has preserved the FDs and the association between report_no and author_no that was important in the original table. Data for the three tables is shown in Figure 6.3. The FDs for these 2NF tables are reportl report_no -
