Database Systems ECS 165B
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This 1 page Class Notes was uploaded by Ashleigh Dare on Tuesday September 8, 2015. The Class Notes belongs to ECS 165B at University of California - Davis taught by Bertram Ludaescher in Fall. Since its upload, it has received 48 views. For similar materials see /class/187775/ecs-165b-university-of-california-davis in Engineering Computer Science at University of California - Davis.
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Date Created: 09/08/15
ECS 165B Database Systems Extended Entity Relationship Model EER Model The objective of the EER model is to make some objectoriented aspects more explicit in particular the modeling of complex attributes associated with entity types Object valued attribute The rst diagram shows an object valued attribute associated with the entity type El Each entity of type El has exactly one attribute of type E2 in addition to perhaps some other attributes An entity of type E2 must always be associated with at least one entity of type El existence dependency In the plain ER model this aspect would be modeled as E l E shown in the second diagram 0 1 m Com plex valued attributes There are several types of complexvalued attributes that can be associated with an entity type Set valued attribute Here an entity of type El has a set of entities of type E2 as single attribute For example a person has a set of addresses each address would be an entity on its own As above the existence of an entity of type E2 depends on whether it is part of an attribute value of an entity of type 1 E 0 The same can be modeled in the plain ER model as shown 05 on the right E List valued attribute 2 E Here an entity of type El has a list of entities of type E2 as single attribute For example a book has a list of authors A hSt 0f I each author would be an entity on its own As above the existence of an entity of type E2 depends on whether it is part of an attribute value of an entity of type El E Again the same can be modeled in the ER model as shown on the right 03 The case for a bagvalued attribute would be the same except that in the plain ER model one would name the relationship type attribute occurrences Note about Cardinality Constraints With a complex valued attribute cardinality constraints can be associated specifying how many members there have to be in the set list of bag For example the following notation would 2 specify that the listvalued attribute named A associated with an entity of type Elhas at least 2 members and at most 8 members min at the bottom max at the top Dr Michael Gertz
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