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Later in this chapter, I provide some guidelines about selecting keys (primary keys in particular).
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Step-by-Step: Building an Entity-Relationship Diagram
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You can build logical diagrams by using tools such as the Oracle Designer, or the Oracle Warehouse Builder if you are building a data warehouse. If you wish, you can create rudimentary logical diagrams with nothing more than a pencil and paper. In this section, you ll build a simple entity-relationship diagram describing a university, using entities called Student, Class, and Professor. You ll use a rectangle to depict an entity, and a diamond shape to show relationships (as is common practice), although you could use different notations. Let s assume the following relationship between two entities, Student and Class: A Student can enroll in one or more Classes. A Class has one or more Students enrolled. Data modeling starts out easy and then rapidly gets complex as you begin to ask questions and discover the various rules and constraints in force on the data. Here are the steps you need to follow to create the entity-relationship diagram: 1. Define your entities Student, Class, and Professor. 2. Draw the entities using a rectangle to represent each one. 3. For each of the entities in turn, look at its relationship with the others. It doesn t matter which entity you begin with. For example, look at the Student and the Professor. Is there a relationship between these entities Well, a Professor teaches a class, and a student attends one or more classes, so at first glance there is a relationship between these entities. But in this case it is an indirect relationship via the Class entity. 4. Examine the Student and Class entities. Is there a relationship Yes, a Student may attend one or more Classes. One or more Students may attend a Class. This is a many-to-many relationship. 5. Now look at the Class and Professor entities. One Professor teaches each Class and each Professor can teach many Classes. However, if a Professor is absent (due to illness, for example), do you need to record the fact that a different Professor taught his or her Class on that occasion What if two Professors teach the same Class Do you need to record that information As a modeler, you need to address all questions of this nature so that your model is realistic and will serve you well. 6. Assign the following attributes to the various entities: Student: Student ID, First Name, Last Name, Address, Year Professor: Staff ID, Social Security Number, First Name, Last Name, Office Address, Phone Number Class: Class ID, Classroom, Textbook, Credit Hours, Class Fee Look at the Textbook attribute in the Class entity. You can use this attribute to illustrate an important point. As the entity stands right now, you could assign only one Textbook per Class. This could be the case, depending on the business rules involved, but what do you do if you need to record the fact that there are several textbooks recommended for each Class The current model would not permit you to do this unless you stored multiple data items in a single field. To resolve this, you could add a new entity called Textbooks, which could then be related to the Class entity. This way, you could associate many different Textbooks with each Class.
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CHAPTER 2 RELA TION AL DA TA BAS E MODE LING AND DATA BASE DESIGN
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7. The cardinality of a relationship, as you saw earlier, dictates whether a relationship is one-toone, one-to-many, many-to-many, or something else. Define the cardinality of the system at this point. Assign the one-to-many or many-to-one cardinality indicators. Break down any many-to-many relationships to simpler relationships (such as two one-to-many relationships). For example: A Student can enroll in one or more Classes. Each Class can have many Students enrolled. This is a many-to-many relationship, which you must break down by using a link table. In this case, the link table turns out to be an entity in its own right. This new entity contains the individual enrollment record for each Class attended by a single Student. 8. Translate the relationships into an actual entity-relationship diagram by using rectangles for entities, diamonds for relationships, and ovals for the attributes of the entities. Your entity-relationship diagram should be able to address all the functional requirements of the database in order for it to be adopted as a valid model. In the preceding example, I used some straightforward relationships among the various entities, but in real life, you may encounter more complex relationships like the recursive relationship, when data within an entity has a relationship to itself. For example, in a Staff table, a member of the staff may report to a higher level member of the staff. If this is the case, then the table is said to have a recursive relationship with itself. I have barely scratched the surface of ER modeling, which is an art in itself one at which you will improve with practice. As with anything else, the more time you spend actually practicing data modeling, the more proficient you will get at it.
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Tip The Internet is a great source for both simple and complex case studies you can use to try out your modeling skills. You can find anything from simple order processing databases to full-fledged personnel systems on the Web. One of the best resources I ve found is the web sites of major universities. Find the descriptions of computer science courses and pay special attention to the contents of database design courses, many of which have tutorials on creating entity-relationship diagrams.
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