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CA218 Introduction to Databases Notes

Chapter 4: Entity-Relationship (ER) Data Modelling
• • • • • Introduction ER Definitions ER Notation Relationships ER Examples

(c) Martin Crane 2011

CA218 Introduction to Databases Notes

Introduction to ER Modelling
• • • • • An Entity-relationship model (ERM) is an abstract and conceptual representation of data. ER modelling is a DB modelling method, used to produce a type of conceptual schema of a system. Diagrams created by this process are called ER diagrams. Sequence: Conceptual data model (i.e. ER) is, at a later stage (called logical design), mapped to a logical data model, (e.g. relational model); this is mapped to a physical model in physical design. ER Model used to interpret, specify & document requirements for DBs irrespective of DBMS being used.

Planning/ Analysis

Data Model (ER Diagram)

Logical DB Conceptual Design

Logical Data Model (Relational, Normal Form)

Physical DB Design

Physical Data Model (Tables with P., F. Keys etc)

Implementation

(c) Martin Crane 2011

CA218 Introduction to Databases Notes



Entity (Instance):

ER Definitions
Student

– An instance of a physical object in the real world. – Entity Class: Group of objects of the same type. – E.g. Entity Class “Student”, Entities “John”, “Trish” etc



Attributes:
– Properties of Entities that describe their characteristics. – Types: • Simple: Attribute that is not divisible, e.g. age. • Composite: Attribute composed of several simple attributes, e.g. address (house number, street, district) • Multiple : Attribute with a set of possible values for the same entity, e.g. Phone (home, mobile etc.) or email • Key: Uniquely Ids the Entity e.g. PPSN, Chassis No. – Value Set (or domain): Each simple attribute associated with a VS that may be assigned to that attribute for each individual entity, e.g. age =...

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