September 6, 2017 Sam Siewert

CS317File and Database Systems

Lecture 2 – Basic ER ModelsPart-2

http://dilbert.com/strips/comic/2012-08-14/

Reminders…Assignement #1, Due Today

Late Due Date (10% penalty) is Monday

Assignment #2, Available on Sunday Morning

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DBMS Analysis and Design

Basic ER-Models

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For Discussion…SA/SD – Focused on Dataflow and Structure

1. SA/SD Included a Data Dictionary which is Data on Data (Meta-data) that describes DBMS Tables (the Schema)

2. Entities in a DBMS ERD Contain Meta-data

3. Chen’s Goal in 1976 with the Entity Relationship Model Was to Show the Structure of Relations and Cardinality

4. Work in DBMS at the time Was Normalization of Relational Models (Elimination of Redundant Data in Tables, Related through Foreign Keys)

5. Compare to Relational Model, Network Model, Hierarchical Model (pages 46-49 in Connolly-Begg - Scanned and Uploaded to Canvas)

6. UML – ER Model comparable to UML Class Diagram (not identical), EER (Enhanced ER Model) for Superclass and Subclass Relationships

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DBMS Development Lifecycle

ER Models - DBMS Chapter 2

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DreamHome ER ModelCompare to DreamHome Relational Model

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Tables

1) Staff2) Branch3) Registration

(Ternary Registers)

• Lease4) Client• Preference5) PropertyForRent• Newspaper6) PrivateOwner• Business Owner7) Viewing

(Conceptual)

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Examples of Entity Types

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Semantic net of Has relationship type

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ER diagram of Branch Has Staff relationship

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Relationship Types

Degree of a Relationship– Number of participating entities in relationship.

Relationship of degree :– two is binary – three is ternary– four is quaternary.

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Ternary relationship called Registers

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Quaternary relationship called Arranges

E.g. Staff arranges a Viewing on behalf of a PrivateOwner supported by a Branch

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ER diagram of Staff and Branchentities and their attributes

DreamHome SchemaWorking Example DB from old Textbook

Can Create from DreamHome version 1.0

Used in Examples in Class

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Strong entity type called Client and weak entity type called Preference

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Relationship called Advertises with attributes

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Structural Constraints

The most common degree for relationships is binary.

Binary relationships are generally referred to as being:– one-to-one (1:1)– one-to-many (1:*)– many-to-many (*:*)

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Semantic net of Staff Manages Branchrelationship type

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Multiplicity of Staff Manages Branch(1:1) relationship

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Semantic net of Staff Oversees PropertyForRent relationship type

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Multiplicity of Staff OverseesPropertyForRent (1:*) relationship type

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Semantic net of Newspaper AdvertisesPropertyForRent relationship type

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Multiplicity of Newspaper AdvertisesPropertyForRent (*:*) relationship

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Semantic net of ternary Registersrelationship with values for Staff and

Branch entities fixed

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Multiplicity of ternary Registersrelationship

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Summary of multiplicity constraints

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Multiplicity as cardinality and participation constraints

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Problems with ER Models

Fan Trap– Where a model represents a relationship between entity types, but

pathway between certain entity occurrences is ambiguous.

Chasm Trap– Where a model suggests the existence of a relationship between

entity types, but pathway does not exist between certain entity occurrences.

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An Example of a Fan Trap

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Semantic Net of ER Model with Fan Trap

At which branch office does staff number SG37 work?

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Restructuring ER model to remove Fan Trap

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Semantic Net of Restructured ER Model with Fan Trap Removed

SG37 works at branch B003.

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An Example of a Chasm Trap

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Semantic Net of ER Model with Chasm Trap

At which branch office is property PA14 available?

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ER Model restructured to remove Chasm Trap

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Semantic Net of Restructured ER Model with Chasm Trap Removed