E-R Model (2) IS 240 – Database Management Lecture #6 – Prof. M. E. Kabay, PhD, CISSP Norwich University

2 Copyright © 2004 M. E. Kabay. All rights reserved. Topics  Diagrams  Weak entities  Subtype entities  Developing the E-R model  Modeling user perceptions  Homework

3 Copyright © 2004 M. E. Kabay. All rights reserved. Diagrams EntityClass Maximum cardinality Entity1 HAS A Entity2 M:N Relationship Entity1–Entity2 1:N Crow’s feet N:1

4 Copyright © 2004 M. E. Kabay. All rights reserved. Diagrams (cont’d) 1:N Entity1–Entity2 Entity1 must have at least one Entity2 Entity2 may have No Entity1; or Entity2 must have 0 or 1 Entity1 1:N Entity3 Entity3 may include no or up to N other Entity3

5 Copyright © 2004 M. E. Kabay. All rights reserved. Diagrams (cont’d)  How can you create E-R diagrams?  CASE tools  PowerPoint  Other drawing programs  PowerPoint tools

6 Copyright © 2004 M. E. Kabay. All rights reserved. Hints for Working with PowerPoint  Use SlideView to access entire screen  Draw a symbol  set its line width, fill, and color  SET AUTOSHAPE DEFAULTS using right- click menu  Drag Flowchart and Connectors menus out of the AutoShapes menu – place at one side of screen  Use the Align or Distribute menu to arrange symbols

7 Copyright © 2004 M. E. Kabay. All rights reserved. Strong and Weak Entities  Strong entity can exist in a DB even if no other entities are linked to it; e.g.,  Customer may not have purchase  Student may have no classes  Professor may have no classes  Weak entity cannot exist in DB unless another entity exists; e.g.,  Purchase cannot exist without a customer and without an item  Class may not exist without a student or without a professor

8 Copyright © 2004 M. E. Kabay. All rights reserved. Weak Entities  Weak entities are shown in diagrams using a rounded-corner rectangle  ID-dependent entities  Compound identifier [entity1, entity2]; e.g., [Course,Section] [Class, TestNumber] [AutoMaker, PartNumber]  Differences of usage for term  Some designers look only at DB structure  Others look at logical relationships (see p. 57 for more detail)

9 Copyright © 2004 M. E. Kabay. All rights reserved. Subtype Entities  What if an entity has optional characteristics? E.g.,  Gender-specific organs and diseases  Social security number when customer is a company vs an individual  WheelDrive or Transmission for auto vs bicycle in Vehicle dataset  If all the possible attributes are defined for an entity-class, some attributes (fields) will be null  Can define subtype entities  Truck, auto, tractor, motorcycle, bicycle...  Male, female  Adult, child  Individual, company

10 Copyright © 2004 M. E. Kabay. All rights reserved. Diagramming Subtypes

11 Copyright © 2004 M. E. Kabay. All rights reserved. Showing Properties  Diagrams can show attributes (properties) of entities  See Fig 3-6, p. 56 for two styles (and next slides)  Each attribute shown in an oval attached to entity box or relationship diamond  Or provide a list of attributes in a rectangular table with text labels

12 Copyright © 2004 M. E. Kabay. All rights reserved. Properties Diagrams (1)

13 Copyright © 2004 M. E. Kabay. All rights reserved. Properties Diagrams (2)

14 Copyright © 2004 M. E. Kabay. All rights reserved. Developing the E-R Model  Designer must understand what the users do and want to do with the system  Include detailed analysis of business rules  Constraints on relationships; e.g., What determines if a professor is assigned a class? Are there prerequisites for classes? What are the prerequisites for a specific class?  Are there exceptions to these rules? Which?

15 Copyright © 2004 M. E. Kabay. All rights reserved. Modeling User Perceptions  Remember that the DB is not intended to model reality in the way a scientific model models the observable, objective world  Dealing with how human beings run their business  Analysts’ perceptions are secondary to users’ way of doing their work  But beware of paving the cowpath  If there is a better way of doing the job by using the DB, discuss it  E.g., just because a handwritten report uses old pre-printed forms does not mean that the DB system should use them

16 Copyright © 2004 M. E. Kabay. All rights reserved. Homework  For next Tueday, 17 Feb 2004  Study and review Chapter 3, pp  Pay particular attention to Examples 1 & 2 (Dance Club, Sailboat Charters)  For 14 points, answer questions and use PowerPoint or other computer- based tool to create your diagrams  By Tuesday, 24 Feb 2004, for 1 extra point each, do any or all of  Problems (up to 4 points)  Project A  Firedup Project questions A-E (up to 5 points)

17 Copyright © 2004 M. E. Kabay. All rights reserved. DISCUSSION