3024.S08.HW3.KEY

3024.S08.HW3.KEY - ISE 3024/S08 HW3 KEY 1 of 4 ISE 3024...

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ISE 3024/S08: HW3 KEY - 1 of 4 ISE 3024 Data Management for IEs Spring 2008 Homework Assignment #3 Solution Key (10) 1. a) Primary key = set of one or more attributes which uniquely define entities in a class. There will be as many primary keys as there are entity classes in a given conceptual schema. (b) Single attribute, value unlikely to change. (c) Specify a secondary key , based upon attributes the user is familiar with. Example: cable bill has a customer ID # (primary key), but customers can also specify their last name and address. (9) 2. A null value is when nothing is assigned to an attribute (i.e., empty space exists) for a particular entity. These are not desirable . This is because they create ambiguity . A null value could mean either the attribute is not applicable for that entity, or the value is known but missing (not recorded), or • the value is unknown. (8) 3. Weak entity class = entity class which can t exist on it s own (i.e., weak entity can only exist through relationship with some strong entity).
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