Database Modeling & Design Fourth Edition- P15 pptx

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 “division- address” 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 identify- ing 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, highest-degree and jobtitle Teorey.book Page 57 Saturday, July 16, 2005 12:57 PM 58 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling • Technician, with identifier empno, and descriptors empname and specialty • Secretary, with identifier empno, and descriptors empname and best-skill Let’s say we determine, through our analysis, that the entity Employee could be created as a generalization of Manager, Engineer, Technician, and Secretary. Then we put identifier empno and generic descriptors empname, address, and date-of-birth in the supertype entity Employee; identifier empno and specific descriptor jobtitle in the subtype entity Manager; identifier empno and specific descriptor highest- degree and jobtitle in the subtype entity Engineer, etc. Later, if we decide to eliminate Employee as a table, the original identifiers and generic attributes can be redistributed to all the subtype tables. (Note that we put table names in boldface throughout the book for readability.) 4.3.3 Define Relationships We now deal with data elements that represent associations among entities, which we call relationships. Examples of typical relationships are “works-in,” “works-for,” “purchases,” “drives,” or any verb that connects entities. For every relationship, the following should be specified: degree (binary, ternary, etc.); connectivity (one-to-many, etc.); optional or man- datory existence; and any attributes associated with the relationship and not the entities. The following are some guidelines for defining the more difficult types of relationships. Redundant Relationships Analyze redundant relationships carefully. Two or more relationships that are used to represent the same concept are considered redundant. Redundant relationships are more likely to result in unnormalized tables when transforming the ER model into relational schemas. Note that two or more relationships are allowed between the same two entities, as long as those relationships have different meanings. In this case they are not considered redundant. One important case of nonredundancy is shown in Figure 4.1a for the ER model and Figure 4.1c for UML. If “belongs-to” is a one-to-many relationship between Employee and Professional-association, if “located-in” is a one-to-many relationship between Profes- sional-association and City, and if “lives-in” is a one-to-many relation- Teorey.book Page 58 Saturday, July 16, 2005 12:57 PM 4.3 Conceptual Data Modeling 59 Figure 4.1 Redundant relationships Employee Employee Professional-association (a) Nonredundant relationships (b) Redundant relationships using transitivity Project located-in located-in City City lives-in works-in belongs-to works-on N N N N N N 1 1 1 1 1 1 Employee Employee Professional-association (c) Nonredundant associations (d) Redundant associations using transitivity Project located-in located-in City City lives-in works-in belongs-to works-on * * * * 1 * * 1 1 1 1 1 Teorey.book Page 59 Saturday, July 16, 2005 12:57 PM 60 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling ship between Employee and City, then “lives-in” is not redundant, because the relationships are unrelated. However, consider the situation shown in Figure 4.1b for the ER model and Figure 4.1d for UML. The employee works on a project located in a city, so the “works-in” relationship between Employee and City is redundant and can be eliminated. Ternary Relationships Define ternary relationships carefully. We define a ternary relationship among three entities only when the concept cannot be represented by several binary relationships among those entities. For example, let us assume there is some association among entities Technician, Project, and Notebook. If each technician can be working on any of several projects and using the same notebooks on each project, then three many-to-many binary relationships can be defined (see Figure 4.2a for the ER model and Figure 4.2c for UML). If, however, each technician is Figure 4.2 Ternary relationships Technician (a) Binary relationships Project has Notebook uses works-on N N N N N N Technician (b) Different meaning using a ternary relationship Project uses- notebook Notebook 11 1 Technician Project Notebook (c) Binary associations * * * ** * works-on has Technician Project Notebook (d) Different meaning using a ternary association ** * uses-notebook uses Teorey.book Page 60 Saturday, July 16, 2005 12:57 PM 4.3 Conceptual Data Modeling 61 constrained to use exactly one notebook for each project and that notebook belongs to only one technician, then a one-to-one-to-one ternary relationship should be defined (see Figure 4.2b for the ER model and Fig- ure 4.2d for UML). The approach to take in ER modeling is to first attempt to express the associations in terms of binary relationships; if this is impossible because of the constraints of the associations, try to express them in terms of a ternary. The meaning of connectivity for ternary relationships is important. Figure 4.2b shows that for a given pair of instances of Technician and Project, there is only one corresponding instance of Notebook; for a given pair of instances of Technician and Notebook, there is only one corresponding instance of Project; and for a given pair of instances of Project and Notebook, there is only one instance of Technician. In gen- eral, we know by our definition of ternary relationships that if a relationship among three entities can only be expressed by a functional depen- dency involving the keys of all three entities, then it cannot be expressed using only binary relationships, which only apply to associations between two entities. Object-oriented design provides arguably a better way to model this situation [Muller, 1999]. 4.3.4 Example of Data Modeling: Company Personnel and Project Database ER Modeling of Individual Views Based on Requirements Let us suppose it is desirable to build a company-wide database for a large engineering firm that keeps track of all full-time personnel, their skills and projects assigned, the departments (and divisions) worked in, the engineer professional associations belonged to, and the engineer desktop computers allocated. During the requirements collection pro- cess—that is, interviewing the end users—we obtain three views of the database. The first view, a management view, defines each employee as working in a single department, and defines a division as the basic unit in the company, consisting of many departments. Each division and department has a manager, and we want to keep track of each manager. The ER model for this view is shown in Figure 4.3a. The second view defines each employee as having a job title: engineer, technician, secretary, manager, and so on. Engineers typically belong to professional associations and might be allocated an engineer- Teorey.book Page 61 Saturday, July 16, 2005 12:57 PM . entities. Object-oriented design provides arguably a better way to model this situation [Muller, 1999]. 4.3.4 Example of Data Modeling: Company Personnel and Project Database ER Modeling of Individual. “division- address” 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. Page 57 Saturday, July 16, 2005 12:57 PM 58 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling • Technician, with identifier empno, and descriptors empname and specialty • Secretary,