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E-R model

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Entity-relationship model

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Entity-relationship model

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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.

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ERD

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Entity-relationship diagram

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Entity-relationship diagram

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A graphical representation of an entity-relationship model.

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Modeling the Rules of the Organization

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In fact, documenting rules and policies of an organization that govern data is exactly what data modeling is all about.

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Business rule

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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.

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A business rules approach is based on the following premises:

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Core concept Familiar to end users. Highly maintainable Automated

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Characteristics of a Good Business Rule

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Declarative Precise Atomic Consistent other rules. Expressible Distinct Business-oriented

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Declarative

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Describes what a process validates.

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Precise

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The rule must have only one interpretation and must be clear.

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Atomic

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Stand on its own as a rule

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Consistent

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Must be consistent with other rules.

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Expressible

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Stated in a structured natural language so that there is no misinterpretation

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Distinct

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Business rules are not redundant, but a business rule may refer to other rules

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Business-oriented

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A business rule is stated in terms businesspeople can understand

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Business Rules Data Naming

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• 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

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Business Rules Data Definition

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Is an explanation of a term or a fact.

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Term

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A word or phrase that has a specific meaning for the business.

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Fact

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An association between two or more terms.

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Good Data Definition Three alternatives:

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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.

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Entity

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A person, a place, an object, an event, or a concept in the user environment about which the organization wishes to maintain data.

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Entity type

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A collection of entities that share common properties or characteristics.

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Entity instance

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A single occurrence of an entity type.

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Strong entity type

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An entity that exists independently of other entity types.

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Weak entity type

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An entity type whose existence depends on some other entity type.

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Identifying owner

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The entity type on which the weak entity type depends.

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Identifying relationship

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The relationship between a weak entity type and its owner.

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Guidelines for naming entity types

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Singular noun Specific to the organization Concise Abbreviation, or a short name Entity types Name should be the same

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Guidelines for defining entity types

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• 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

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Attribute

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A property or characteristic of an entity or relationship type that is of interest to the organization.

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Required attribute

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An attribute that must have a value for every entity (or relationship) instance with which it is associated.

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Optional attribute

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An attribute that may not have a value for every entity (or relationship) instance with which it is associated

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Composite attribute

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An attribute that has meaningful component parts (attributes).

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Simple (or atomic) attribute

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An attribute that cannot be broken down into smaller components that are meaningful to the organization.

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Multivalued attribute

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An attribute that may take on more than one value for a given entity (or relationship) instance

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Derived attribute

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An attribute whose values can be calculated from related attribute values.

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Identifier

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An attribute (or combination of attributes) whose value distinguishes instances of an entity type.

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Composite identifier

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An identifier that consists of a composite attribute.

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Criteria for selecting identifiers

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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

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Guidelines for Naming Attributes

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Singular noun or noun phrase Unique Each attribute name should follow a standard format Similar attributes should use the same qualifiers and classes,

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Guidelines for Defining Attributes

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>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

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Relationship

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An association representing an interaction among the instances of one or more entity types that is of interest to the organization.

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Relationship type

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A meaningful association between (or among) entity types.

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Relationship instance

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An association between (or among) entity instances where each relationship instance associates exactly one entity instance from each participating entity type.

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Associative entity

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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.

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How do you know whether to convert a relationship to an associative entity type? Following are four conditions that should exist:

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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.

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Degree

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The number of entity types that participate in a relationship.

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Degree of a Relationship

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The number of entity types that participate in a relationship.

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What are three most common relationship degrees in E-R models?

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Unary (degree 1), binary (degree 2), and ternary (degree 3).

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Unary relationship

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A relationship between instances of a single entity type.

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Binary relationship

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A relationship between the instances of two entity types.

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Ternary Relationship

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A ternary relationship is a simultaneous relationship among the instances of three entity types.

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Cardinality constraint

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A rule that specifies the number of instances of one entity that can (or must) be associated with each instance of another entity.

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Types of Cardinality Cosnstraints

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Minimum cardinality Maximum cardinality

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Minimum cardinality

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The minimum number of instances of one entity that may be associated with each instance of another entity.

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Maximum cardinality

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The maximum number of instances of one entity that may be associated with each instance of another entity.

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Time stamp

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A time value that is associated with a data value, often indicating when some event occurred that affected the data value.

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Guidelines for naming relationships

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Verb phrase Avoid vague names

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Guidelines for defining relationships

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> 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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