Database Modeling & Design Fourth Edition- P18: 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. | 72 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling a Schema 3 the result of merging schema 1 and schema Figure View integration the merged schema At this point we have sufficient commonality between schemas to attempt a merge. In schemas 1 and we have two sets of common entities Department and Topic-area. Other entities do not overlap and must appear intact in the superimposed or merged schema. The merged schema schema 3 is shown in Figure . Because the common entities are truly equivalent there are no bad side effects of the merge due to existing relationships involving those entities in one schema and not in the other. Such a relationship that remains intact exists in schema 1 between Topic-area and Report for example. If true equivalence cannot be established the merge may not be possible in the existing form. In Figure there is some redundancy between Publication and Report in terms of the relationships with Department and Topic-area. Such a redundancy can be eliminated if there is a supertype subtype relationship between Publication and Report which does in fact occur in this case because Publication is a generalization of Report. In schema Figure we see the introduction of this generalization from Report to Publication. Then in schema Figure we see that the View Integration 73 b Schema new generalization c Schema elimination of redundant relationships Figure continued 74 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling redundant relationships between Report and Department and Topic-area have been dropped. The attribute title has been eliminated as an attribute of Report in Figure because title already appears as an attribute of Publication at a higher level of abstraction title is inherited by the subtype Report. The final schema in Figure expresses completeness because all the original concepts report publication topic area department and contractor are kept intact. It .
