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Database Conceptual Design using Entity Relationship Diagram CIS 8040 Database Management Systems J.G. Zheng Feb 2010

Overview

Conceptual modeling/design Using Entity Relationship Diagram (ERD)  Basic  Extended Design/modeling considerations Data dictionary 2

Three Level Database Design

Conceptual design  Using Entity Relationship Diagram (ERD) to represent the reality and business data requirements Logical design Physical design 3

Conceptual design

Conceptual modeling/data modeling Usually using diagram to visually represent the model  ERD 4

Entity-Relationship Model

Proposed by Dr. Peter Chen in 1970s  http://en.wikipedia.org/wiki/Peter_Chen ERD is a conceptual model Major elements   Entity (with attributes and identifier) Relationship 5

Entity and Attribute

Entity   Entity class (entity set) Entity instance Attribute Identifier 6

Entity Notation in ERD

Note: there are several variations of ERD fine – just be consistent.

Modeling Consideration 1

Attribute, attributes or entity?

       “Person Name” “Transaction Date” “Address” “ZIP” “State” “Department” “Skill” 8

Modeling Consideration 2

Attribute or attribute values?

How to model the following scenario?

  Contacts  An sales person can be contacted by “Fax Number”, “Cell Phone Number”, “Home Phone Number”, “Work Phone Number”, “Work Email”, etc.

Advising Hours  Faculty members have specific advising hours on 5 week days: “Monday”, “Tuesday”, etc.

Entity-attribute-value model  http://en.wikipedia.org/wiki/Entity-attribute-value_model 9

ERD Exercise 1

Draw a ERD (only entities and attributes) about movies Sample data Movie: Director: Release Date: Genre: Production: Actor/ess: Mission Impossible Brian De Palma 22 May 1996 Action Paramount Pictures (USA) Tom Cruise (male), Jon Voight (male) 10

Relationship

Degree of relationship  How many entities are involved?

Binary (two entities) relationship is most common Books publishes Publishers 11

Unary Relationship

Unary (recursive) – only 1 entity Employee Customer ForumPost Refers Supervises replies 12

Ternary Relationship

Ternary – 3 entities 13

N’ary Relationship

4 entities Physician Patient Operates Supplies Nurse 14

Modeling Consideration 3

Multiple independent relationships?

Faculty Teaches Student Advises Supervises Employee Mentors 15

Modeling Consideration 4

3 binary relationships or a ternary one?

 Database Modeling and Design: Logical Design, 4th Edition by Toby J. Teorey, Sam S. Lightstone, and Tom Nadeau, 2005 16

Modeling Consideration 4 (continued) 3 binary relationships or a ternary one?

Movie Act in Actor/ess has Movie Character Act as … in … Character Act as Actor/ess 17

Types of Relationship (1)

One-to-One (1:1)  A single entity instance in one entity class is related to a single entity instance in another entity class ER Notation (Crow’s foot) Governor governs State 18

Types of Relationship (2)

One-to-Many (1:N)  A single entity instance in one entity class ( is related to multiple entity instances in another entity class ( child ) parent ) ER Notation (Crow’s foot) Books publish Publishers 19

Types of Relationship (3)

Many-to-Many (N:M)  Each entity instance in one entity class is related to multiple entity instances in another entity class; and vice versa ER Notation (Crow’s foot) Books write Authors 20

Cardinality

Cardinality  Describes participation in the relationship Maximum cardinalities (types of relationships) Minimum cardinalities   describes the minimum number of instances that must participate in a relationship Optional (zero) or Mandatory (one) Certificates have Programmers 21

ERD Exercise 2

Movie data continued … Let’s only consider the following entities and their attributes    Perfomers: PerformerID, FirstName, LastName, Gender Movies: MovieID, Title, Maker, Year MovieMakers(companies): MakerID, Name Assumptions (business rules)     A movie has at least one actor/actress An actor/actress does not have to be in any movie A company does not have to make any movie A movie does not have to be made by a company 22

Modeling Consideration 5

Attributes of a relationship?

Modeling Consideration 6

Relationship as an entity?

Movie Acts in Actor/ess Movie Cast Actor/ess 24

ERD Exercise 3

Movie data continued … Now considering this  Tom Cruise acts as Ethan Hunt in “Mission Impossible”.

Let’s consider more information about “Ethan Hunt”  “Ethan Hunt”: male, fiction figure, agent, etc.

ERD: Extended

More on entity  Strong vs. weak entity More on relationship  Generalization 26

Weak Entity

The existence of such entity has no real business (logical) meaning without another entity (owning entity)  Requires the participation from the owning entity (minimum cardinality is 1) Employee has Dependent Building has Room 27

ID Dependency

Weak entity’s identifier relies on its owning entity (or indentifying entity)’s identifier Building Building Id has Room Building Id Room Number 28

Modeling Consideration 7

Weak entity or not?

 Many argues that weak entity is not important in database modeling Building Building Id has Room Building Id Room Number Building Room Building Id has Room Number 29

Super and Sub Types

A Sub-type is a special case, or a category, of a Super-type    Student : Graduates, Undergraduates Employee : full-time, part-time, contractor Customer : individual, company, non-profit 30

Sub-Type Completeness

Completeness  Do all super type instances appear in any sub-type entities?

Participant Employee Organization Individual Student Employee 31

Sub-Type Disjointness

Disjointness: does any instance appear in multiple subtype entities?

  Yes: No: Overlap (Inclusive) Disjoint (Exclusive) 32

Modeling Consideration 8

When to use sub-type (specification)?

 There are attributes that apply to some (but not all) of the instances of an entity type 33

Modeling Consideration 8 (continued) When to use (specification)?

 The instances of a subtype participate in a relationship unique to that subtype 34

Modeling Consideration 9

When to use super-type (generalization)?

  When several entities have same major attributes, see if they are constantly treated together.

  Faculty, staff, student assistant Employee, customer, business partner (or, supplier) Multiple entities participate in the same relationship with the same entity (also see modeling trap 3) Donation Made by Organization Made by Individual 35

ERD Exercise 4

Type of people in GSU       Student: full time, part time, graduate, undergraduate, current, prospective, etc.

Student employee: student assistant, GRA, GTA, etc.

Faculty: tenured, tenure track, visiting, PTI, non-teaching Staff: full time, part time, admin Alumni Other: parents, family member, emergency contact, etc.

Modeling Trap 1

Avoid the Fan Trap Ambiguous (broken) relationship between Department and Staff 37

Modeling Trap 2

Avoid the Chasm Trap  See modeling consideration 3 Ambiguous (broken) relationship between Branch and Property 38

Modeling Consideration 10

Redundant relationship?

 A relationship is redundant if it is represented by alternate transitive relationships Department admits Student offers Program admits 39

Modeling Consideration 10 (continued) From: Database Modeling and Design: Logical Design, 4th Edition by Toby J. Teorey, Sam S. Lightstone, and Tom Nadeau, 2005 40

Modeling Trap 3*

Avoid the same (identical) relationship with multiple entities  See modeling consideration 10 Donation Made by Organization Made by Individual 41

Analysis and Modeling Tips

Modeling is an iterative refinement process     Start with basic and obvious facts or concepts; then let entities, attributes, and relationships eventually emerge.

Identify binary relationships and maximum cardinality first.

Identify minimum cardinality and check for common modeling traps; consider n’ery relationships if necessary; check for redundant relationships and missing relationships.

Add, combine, or split entities and attributes as needed. Check relationships and constraints after changes.

Start with specific function areas (user views) and integrate them later  View integration Ensure the consistency between requirements, ERD and the data dictionary 42

Data Dictionary

A document that records detailed descriptions of data requirements and all ERD elements Practically, it should include:   Definitions (detailed description) for entities, attributes and relationships Business rules (constraints, assumptions or other requirements) and justifications that support the design of entities, attributes and relationships  Data type, domain values, etc. – see database project deliverable #1 43

Key Concepts

ERD Entity  Strong vs. weak entity Attribute and identifier Relationship     1:1, 1:N, N:M Degree  Unary, binary, ternary, etc.

Cardinality Super- and sub-types  Completeness and disjointness Data dictionary 44

Summary of Modeling Guidelines

Modeling considerations 1.

10.

Attribute(s) or entity?

Attributes or attribute values?