Intro to Database Week 4 & 5 (midterm)

Entity Relationship Model (ER)

A conceptual data model for database design

Entity & Entity set

Entity: Objects represented in database

Entity set: Collection of actual entities in database

Attribute & Key Attribute

Attribute: Properties of an entity

Key Attribute: unique attribute of an entity (can be composite)

What are the 3 types of attributes

Simple Attribute: entity has 1 value

Composite attribute: attribute has many values (address)

Multi-valued attribute: attribute can have many values

Data value

Value of a specific property

Relationship, relationship type & relationship set

Relationship: relate two or more entities

Relationship type: Description of relationship (participants & constraints)

Relationship set: set of relationship instances in database OR state of relationship type

Degree of Relationships

1. Unary (Recursive)

2. Binary

3. Ternary

4. N-ary relationship

Weak entity

Entity that does not have a key attribute of its own that must be identified by another entry (identifying relationships)

key can be partial key or primary key of identifying (strong) entity

Summary of all the notations

What are the constraints on relationship types (binary)

1. Cardinality

2. Participation Constraint (Existence Dependency)

Cardinality Ratio

Specifying maximum participation

1. One-to-one (1:1)

2. One-to-many (1:N)

3. Many-to-many (M:N)

Participation Constraint

Specifying minimum participation

1. Zero (optional participation, NOT existence dependent

2. One or more (Total participation, existence deponent)

Notation for Relationship Constraints

Relationship attributes appear in

M..N relationships but put at the n-side for 1-N

Each entity must participate at least ____ and at most ____

min, max

an entity does not have to participate if

they have no constraint (0,N) but it can participate at N times

What are the UML similarities to ER

Class = entity

Relationships = relationship

Multiplicities = cardinality

What are the Relationships in Class Diagram

Association: Two independent objects

Aggregation: two related objects, one is part of the other

Link attributes: similar to relationship attributes

Subclasses/Subtypes (Is-A relationship)

Can only be represented by EER (Extended Entity Relation Model)

subclass inherits all attributes & relationships of superclass

Specialization

Defines a set of subclasses of a superclass

Generalization

subclasses are generalized into one superclass