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Database Modeling & Design Fourth Edition- P15

Database Modeling & Design Fourth Edition- P15: 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. | 4.3 Conceptual Data Modeling 57 include Employee Task Project Department Company Customer and so on. Multivalued Attributes Classify multivalued attributes as entities. If more than one value of a descriptor attribute corresponds to one value of an identifier the descriptor should be classified as an entity instead of an attribute even though it does not have descriptors itself. A large company for example could have many divisions some of them possibly in different cities. In that case division could be classified as a multivalued attribute of company but it would be better classified as an entity with divisionaddress as its identifier. If attributes are restricted to be single valued only the later design and implementation decisions will be simplified. Attribute Attachment Attach attributes to the entities they most directly describe. For example office-building-name should normally be an attribute of the entity Department rather than the entity Employee. The procedure of identifying entities and attaching attributes to entities is iterative. Classify some data elements as entities and attach identifiers and descriptors to them. If you find some violation of the preceding guidelines change some data elements from entity to attribute or from attribute to entity attach attributes to the new entities and so forth. 4.3.2 Identify the Generalization Hierarchies If there is a generalization hierarchy among entities then put the identifier and generic descriptors in the supertype entity and put the same identifier and specific descriptors in the subtype entities. For example suppose five entities were identified in the ER model shown in Figure 2.4a Employee with identifier empno and descriptors empname address and date-of-birth Manager with identifier empno and descriptors empname and jobtitle Engineer with identifier empno and descriptors empname high-est-degree and jobtitle 58 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling Technician with identifier .