ER2-tute-soln.pdf - ISYS1055/1057 Tutorial Sheet Solution The Entity-Relationship Model(2 1a “Customers get service from several branches” should

ER2-tute-soln.pdf - ISYS1055/1057 Tutorial Sheet Solution...

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ISYS1055/1057 Tutorial Sheet Solution The Entity-Relationship Model (2) 1a. “Customers get service from several branches” should not be modelled as a relationship in the ER model, otherwise there is data redundancy. All such information is included in the “Service” relationship between Consultant and Customer.
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1b. The ER diagram. The “Service” weak entity set is for the ternary relationship between Branch, Consultant and Customer. 1c. For question (a): - The “Service” relationship does not keep historical data for services between Consultant and Customer. - The description implies that a consultant only services customers at his home branch. So a binary relationship Service is sufficient, and from this relationship it can be inferred the branch where customers receive services. In contrast for question (b): - The “Service” relationship needs to keep historical data. So a weak entity set is needed to keep the service among the (Branch, Consultant, Customer) tuple on different dates. - The “Service” relationship needs to be ternary to explicitly keep data for the relationship among Branch, Consultant and Customer.
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  • Spring '12
  • Boman
  • Entity-relationship model, Relational model, Consultant and Customer

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