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Bus 393 Chapter 2

E-R model
Entity-relationship model
Entity-relationship model
A logical representation of the data for an organization or for a business area, using entities for categories of data and relationships for associations between entities.
ERD
Entity-relationship diagram
Entity-relationship diagram
A graphical representation of an
entity-relationship model.
Modeling the Rules of the Organization
In fact, documenting rules and policies of an organization that govern data is exactly what data modeling is all about.
Business rule
A statement that defines or constrains some aspect of the business. It is intended to assert
business structure or to control or influence the behavior of the business.
A business rules approach is based on the following premises:
Core concept
Familiar to end users.
Highly maintainable
Automated
Characteristics of a Good Business Rule
Declarative
Precise
Atomic
Consistent
other rules.
Expressible
Distinct
Business-oriented
Declarative
Describes what a process validates.
Precise
The rule must have only one interpretation and must be clear.
Atomic
Stand on its own as a rule
Consistent
Must be consistent with other rules.
Expressible
Stated in a structured natural language so that there is no misinterpretation
Distinct
Business rules are not redundant, but a business rule may refer to
other rules
Business-oriented
A business rule is stated in terms businesspeople can understand
Business Rules Data Naming
• Relate to business, not technical (hardware or software), characteristics
• Be meaningful
• Be unique
• Be readable
• Be composed of words taken from an approved list;
• Be repeatable
• Follow a standard syntax
Business Rules Data Definition
Is an explanation of a term or a fact.
Term
A word or phrase that has a specific meaning for the business.
Fact
An association between two or more terms.
Good Data Definition Three alternatives:
1. Use multiple definitions to cover the various situations.
2. Use a very general definition that will cover most situations.
3. Consider using multiple, related, data objects for Student.
Entity
A person, a place, an object, an event, or a concept in the user environment about which
the organization wishes to maintain data.
Entity type
A collection of entities that share common properties or characteristics.
Entity instance
A single occurrence of an entity type.
Strong entity type
An entity that exists independently of other entity types.
Weak entity type
An entity type whose existence depends on some other entity type.
Identifying owner
The entity type on which the weak entity type depends.
Identifying relationship
The relationship between a weak entity type and its owner.
Guidelines for naming entity types
Singular noun
Specific to the organization
Concise
Abbreviation, or a short name
Entity types
Name should be the same
Guidelines for defining entity types
• An entity type definition usually starts with
• Include a statement of what the unique characteristic is for each instance of the entity type
• Entity instances are included and not included
• When an instance of the entity type is created and deleted
• When an instance might change into an instance of another entity type
• What history is to be kept about instances
of the entity type
Attribute
A property or characteristic of an entity or relationship type that is of interest to the organization.
Required attribute
An attribute that must have a value for every entity (or relationship) instance with which it is associated.
Optional attribute
An attribute that may not have a value for every entity (or relationship) instance with
which it is associated
Composite attribute
An attribute that has meaningful component parts (attributes).
Simple (or atomic) attribute
An attribute that cannot be broken down into smaller components that are meaningful to the organization.
Multivalued attribute
An attribute that may take on more than one value for a given entity (or relationship) instance
Derived attribute
An attribute whose values can be calculated from related attribute values.
Identifier
An attribute (or combination of attributes) whose value distinguishes instances of an entity type.
Composite identifier
An identifier that consists of a composite attribute.
Criteria for selecting identifiers
1. Will not change its value over the life
2. Each instance of the entity
3. Avoid the use of so-called intelligent identifiers
4. Single-attribute surrogate identifiers
Guidelines for Naming Attributes
Singular noun or noun phrase
Unique
Each attribute name should follow a standard format
Similar attributes should use the same qualifiers and classes,
Guidelines for Defining Attributes
>What the attribute is and possibly why it is important
>What is included and not included
>Aliases
>The source of values for the attribute
>If a value for the attribute is required or optional
>Whether a value for the attribute may change
>The maximum and minimum number of occurrences of an attribute value for an entity instance
Relationship
An association representing an interaction among the instances of one or more entity types that is of interest to the organization.
Relationship type
A meaningful association between (or among) entity types.
Relationship instance
An association between (or among) entity instances where each relationship instance associates exactly one entity instance from
each participating entity type.
Associative entity
An entity type that associates the instances of one or more entity types and contains attributes that are peculiar to the relationship
between those entity instances.
How do you know whether to convert a relationship to an associative entity type? Following are four conditions that should exist:
1. All the relationships for the participating entity types are “many” relationships.
2. The resulting associative entity type has independent meaning to end users and,
preferably, can be identified with a single-attribute identifier.
3. The associative entity has one or more attributes in addition to the identifier.
4. The associative entity participates in one or more relationships independent of the
entities related in the associated relationship.
Degree
The number of entity types that participate in a relationship.
Degree of a Relationship
The number of entity types that participate in a relationship.
What are three most common relationship degrees in E-R models?
Unary (degree 1), binary (degree 2), and ternary (degree 3).
Unary relationship
A relationship between instances of a single entity type.
Binary relationship
A relationship between the instances of two entity types.
Ternary Relationship
A ternary relationship is a simultaneous relationship among the instances of three entity types.
Cardinality constraint
A rule that specifies the number of instances of one entity that can (or must) be associated with each instance of another entity.
Types of Cardinality Cosnstraints
Minimum cardinality
Maximum cardinality
Minimum cardinality
The minimum number of instances of one entity that may be associated with each instance of another entity.
Maximum cardinality
The maximum number of instances of one entity that may be associated with each instance of another entity.
Time stamp
A time value that is associated with a data value, often indicating when some event occurred that affected the data value.
Guidelines for naming relationships
Verb phrase
Avoid vague names
Guidelines for defining relationships
> Explains what action is being taken and possibly why it is important
> Give examples to clarify the action
> Optional participation
> Explain the reason for any explicit maximum cardinality
> Explain any mutually exclusive relationships
> Explain any restrictions on participation in the relationship
> Explain the extent of history that is kept in the relationship
> Explain whether an entity instance involved in a relationship instance can transfer participation to another relationship instance

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