ER Models ISSUES and Examples CS263 Lecture 9

ER Issues - Fan Trap Where a model represents a relationship between entity types, but the pathway between certain entity occurrences is ambiguous. works for operates Staff Division Branch reports to has Q. At which branch does employee Fred Bloggs (001) work? Staff Division Branch 001 B03 D1 002 B02 D2 003 B04 A. Fred Bloggs (001) works at Branch B03, or is it B02?

ER Issues - Fan Trap Resolution We resolve the Fan Trap by restructuring the original ER model to represent the correct association between the entities concerned. operates has Division Branch Staff reports to work for Q. At which branch does employee Fred Bloggs (001) work? Staff 002 001 003 Division D1 D2 Branch B03 B02 B04 A. Fred Bloggs (001) works at Branch B02

Developing an ER Diagram Step 1 Identify Entities Step 2 Work out relationships Step 3 Identify Attributes Step 4 Identify optionality Step 5 Identify KEY attributes (Resolve any assumptions)

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Example Problem 1 A college library holds books for its members to borrow. Each book may be written by more than one author. Any one author may have written several books. If no copies of a wanted book are currently in stock, a member may make a reservation for the title until it is available. If books are not returned on time a fine is imposed and if the fine is not paid the member is barred from loaning any other books until the fine is paid.

Member Book Author

Title Member Book Author

Title Member Book Author Serial Number. Condition. Date Purchased.

Title EACH Written by Writes Member Borrows Book Author On loan

Title Book Author RESERVES Member BARRED FINE Written by Writes Borrows Book Author On loan BARRED FINE

Title Book Author Fine Member Written Reserves by Reserved By Writes Borrows Book Author On loan MUST MAY Fine

Title Book Author Fine ER Diagram One Member Written Reserves by Reserved By Has Has Writes Member Borrows Book Author On loan Has For Results in Imposed on Fine

ER Diagram Two Title T_A Reserve Member Book Author M_B Fine

Title Author Book Fine Identify Keys T_A Member M_B T# A# M# B# F# Reserve Member M# A# Book Author B# M_B F# Fine

Title Author Book Fine Foreign Keys ER Diagram Two T_A Member T# A# M# Reserve M#,T# Member M# A# Book Author M#,B# B# Borrow T# F# Fine M#,B#

Example Problem 2 A global enterprise has three companies, and each company has between two and six departments. Each department employs one or more employees (although certain company employees may not currently be attached to a department), who, as well as possibly having a number of dependants, may well have an associated set of employment history notes. State any assumptions you make.

Company Department Employee Employment Note Dependent Belongs to Operates Department Employee Employment Note Employs Has Works for Belongs to Belongs to Has Dependent

Example Problem 3 A local authority wishes to keep a database of all its schools and the school children that are attending each school. The system should also be able to record teachers available to be employed at a school and be able to show which teachers teach which children. Each school has one head teacher who’s responsibility it is to manage their individual school, this should also be modelled. State any assumptions you make.

School Teacher Child Employed by Employs Attended by Attends Managed by Manages Teacher Child Teaches Taught by