IS 325

Notes for

Wednesday

September 4, 2013

Syllabus Change

• I eliminated quizzes

• I increased the points allocated to homework assignments

Entity-Relationship Modeling/Diagrams

• Entity-relationship (ER) modeling - conceptual database modeling technique Enables the structuring and organizing of the

requirements collection process Provides a way to graphically represent the

requirements

• ER diagram (ERD) - the result of ER modeling Serves as a blueprint for the database

ENTITIES

• Entities - constructs that represent what the database keeps track of… something of specific interestThe basic building blocks of an ER diagramRepresent various real world notions, such as

people, places, objects, events, items, and other concepts (think “nouns”)

Within an ERD each entity must have a different name and different semantic meaning

ENTITIES

Two entities

ENTITIES

• Entity instances (entity members) - occurrences of an entityEntities themselves are depicted in the ER

diagrams while entity instances are notEntity instances are eventually recorded in the

database that is created based on the ER diagramEach entity instance becomes a row in the

database

ATTRIBUTES

• Attribute - a characteristic of an entityRepresents the details that will be recorded for each

entity instanceWithin one entity, each attribute must have a

different name and different semantic meaning

• Unique Attribute/Set of Attributes – an attribute or set of attributes that have a value(s) is different for each entity instanceEvery regular entity must have at least one unique

attribute or a set of attributes (think George Foreman)

ATTRIBUTES

An entity with attributes

RELATIONSHIPS

• Relationship – meaningful or naturally occurring associations between entitiesWithin an ER diagram, each entity must be related

to at least one other entity via a relationship

RELATIONSHIPS

• Cardinality constraints - depict how many instances of one entity can be associated with instances of another entityMaximum cardinality

One (represented by a straight bar I (Roman Numeral 1))

Many (represented by a crow’s foot symbol)

Minimum cardinality (participation)Zero or Optional (represented by a circular symbol: 0

(Roman Numeral 0))

Mandatory (represented by a straight bar I (roman Numeral 1))

RELATIONSHIPS

A relationship between two entities

RELATIONSHIPS

Four possible cardinality constraints

RELATIONSHIPS

Several possible versions of the relationship "ReportsTo"

RELATIONSHIPS

• Types of Relationships (maximum cardinality-wise)One-to-one relationship (1:1)One-to-many relationship (1:M)Many-to-many relationship (M:N)

RELATIONSHIPS

Three types of relationships (maximum cardinality-wise)

RELATIONSHIPS

1:M Relationship

M:N Relationship

1:1 Relationship

RELATIONSHIPS

• Relationship instances - occurrences of a relationshipOccur when an instance of one entity is naturally

or meaningfully associated to an instance of another entity via a relationship

Relationship themselves are depicted in the ER diagrams while relationship instances are not

Relationship instances are eventually recorded in the database that is created based on the ER diagram

RELATIONSHIPS

A relationship and its instances

RELATIONSHIPS

• Relationship attributes In some cases M:N relationships can actually have

attributes of their own

RELATIONSHIPS

A M:N relationship with an attribute

RELATIONSHIPS

A 1:M relationship with and without an attribute

ER diagram example: ZAGI Retail Company Sales Department Database

ATTRIBUTES

• Composite attribute – attribute that is composed of several attributes (an amalgam or composite)Not an additional attribute of an entityIts purpose is to indicate a situation in which a

collection of attributes has an additional meaning, besides the individual meanings of each attribute

ATTRIBUTES

An entity with a composite attribute

ATTRIBUTES

Another entity with a composite attribute

ATTRIBUTES

Composite attributes sharing components

ATTRIBUTES

• Composite unique attribute – attribute that is composed of several attributes and whose value is different for each entity instance

ATTRIBUTES

An entity with a unique composite attribute

ATTRIBUTES

• Multiple unique single attributes or minimal sets of attributes (candidate keys) - when an entity has more than one unique attribute each unique attribute is also called a candidate key

ATTRIBUTES

An entity with multiple unique attributes (candidate keys)

ATTRIBUTES

An entity with a candidate key and composite key (which does not satisfy the requirements of of a candidate key… I differ with the text authors)

ATTRIBUTES

• Multivalued attribute - attribute for which instances of an entity can have multiple values for the same attribute

ATTRIBUTES

A multivalued attribute

ATTRIBUTES

A scenario that does not use multivalued attributes

ATTRIBUTES

• Derived attribute - attribute whose values are calculated and not permanently stored in a database

ATTRIBUTES

A derived attribute example

ATTRIBUTES

Another derived attribute example

ATTRIBUTES

• Optional attribute - attribute that is allowed to not have a value

ATTRIBUTES

An optional attribute example

ATTRIBUTES

EXAMPLE: An entity with various types of attributes

RELATIONSHIPS

• Exact minimum and maximum cardinality in relationships In some cases the exact minimum and/or maximum

cardinality in relationships is known in advanceExact minimum/and or maximum cardinalities can

be depicted in ER diagrams

RELATIONSHIPS

A relationship with specific minimum and maximum cardinalities

RELATIONSHIPS

RELATIONSHIPS

• Degree of a relationship - reflects how many entities are involved in the relationship

• Binary relationship - relationship between two entities (degree 2 relationship)

• Unary relationship (recursive relationship) - occurs when an entity is involved in a relationship with itself (degree 1 relationship)

RELATIONSHIPS

Unary relationship examples

RELATIONSHIPS

• Relationship roles - additional syntax that can be used in ER diagrams at the discretion of a data modeler to clarify the role of each entity in a relationship

RELATIONSHIPS

Unary relationships with role names

RELATIONSHIPS

A binary relationship with role names

RELATIONSHIPS

• Multiple relationships between same entitiesSame entities in an ER diagram can be related via

more than one relationship

RELATIONSHIPS

Multiple relationships between the same entities

Chalk Talk And Examples

• Let’s practice