Ch4 Entity Relationship Modeling

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ER model

forms the basis of an ER diagram

ERD

represents conceptual database as viewed by end user

ERDs depict database's main components

- Entities
- Attributes
- Relationship

Entities

- Refers to entity set and not to single entity occurrence
- Corresponds to table and not to row in relational environment

Chen and Crow's Foot mode

entity represented by rectangle with entity's name

Attributes

Characteristics of entities

Chen notation

attributes represented by ovals connected to entity rectangle with a line

Crow's Foot notation

attributes written in attribute box below entity rectangle

Required attribute

must have a value

Optional attribute

may be left empty

Domain

set of possible values for an attribute

Identifiers

one or more attributes that uniquely identify each entity instance

Composite identifier

primary key composed of more than one attribute

Composite attribute

can be subdivided

Simple attribute

cannot be subdivided

Single-value attribute

can have only a single value

Multivalued attributes

can have many values

Derived attribute

value may be calculated from other attributes

Relationships

-association between entities
-between entities always operate in both directions
-can be classified as 1:M
-classification is difficult to establish if only one side of the relationship is known

Participants

are entities that participate in a relationship

Connectivity

Describes the relationship classification

Cardinality

Expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity

Connectivity and Cardinality

Established by very concise statements known as business rules

Existence dependence

Entity exists in database only when it is associated with another related entity occurrence

Existence independence

Entity can exist apart from one or more related entities

Relationship Strength

-Weak (non-identifying) relationships
-Strong (identifying) relationships

Weak (non-identifying) relationships

Exists if PK of related entity does not contain PK component of parent entity

Strong (identifying) relationships

Exists when PK of related entity contains PK component of parent entity

Weak Entities

-Existence-dependent
-Primary key partially or totally derived from parent entity in relationship

Relationship Participation

-Optional participation
-Mandatory participation

Optional participation

One entity occurrence does not require corresponding entity occurrence in particular relationship

Mandatory participation

One entity occurrence requires corresponding entity occurrence in particular relationship

Relationship Degree

Indicates number of entities or participants
associated with a relationship
-Unary relationship
-Binary relationship
-Ternary relationship

Recursive Relationships

Relationship can exist between occurrences of the same entity set,naturally found within unary relationship

Associative (Composite) Entities

-Also known as bridge entities
-Used to implement M:N relationships
-Composed of primary keys of each of the entities to be connected
-May also contain additional attributes that play no role in connective process

Developing an ER Diagram

-Create detailed narrative of organization's description of operations
-Identify business rules based on description of operations
-Identify main entities and relationships from business rules
-Develop initial ERD
-Identify attributes and primary keys that adequately describe entities
-Revise and review ERD
(6)

Database Design Challenges

-Database designers must make design compromises
-Important to meet logical requirements and design conventions
-Design of little value unless it delivers all specified query and reporting requirements
-Some design and implementation problems do not yield "clean" solutions
(4)

Conflicting goals

design standards, processing speed, information requirements

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