5. Consider the E-R diagram in Figure 7-20. Based on this E-R diagram, answer th

Question

5. Consider the E-R diagram in Figure 7-20. Based on this E-R diagram, answer the following questions: a. How many EMPLOYEES can work on a project? b. What is the degree of the Used_on relationship? c. Do any associative entities appear in this diagram? If so, name them. d. How else could the attribute Skill be modeled? e. What attributes might be attached to the Works_on relationship? f. Could TOOL be modeled as an associative entity? Why or why not? PROJECT EMPLOYEE Project_ID Employee_ID TOOL TASK CITY Task_ID Time (skill)

Explanation / Answer

An employee can work on one-to-many projects; the Includes relationship is a binary relationship. No associative entities are directly shown by the associative entity symbol. The only many-to-many relationship, Works On, has no attributes, so it does not need to be shown as an associative entity. The SKILL attribute could also be modeled as an attributive or weak entity. It is not possible for the Includes relationship to have attributes because it is a one-to-many relationship. Only one-to-one and many-to-many relationships are allowed to have attributes. TASK could be modeled as an associative entity because it falls at the intersection of mandatory binary relationships between PROJECT, TOOL, and CITY, depending on the meaning of task. As currently modeled, TASK is something done on a project with a tool at a city, so it is not an independent concept. TASK can be modeled as an entity because it has its own primary key independent of the keys of PROJECT, CITY, and TOOL. Some semantic information would be lost if TASK were modeled as an associative entity (e.g., the minimum cardinalities related to task on the Includes, Done at, and Used on relationships)

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