Chapter 8: Conceptual Data Modeling

Focus: What the data is and the rules governing it.

Entity

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

Candidate key

An attribute (or combination of attributes) that uniquely identifies each instance of an entity type.

Identifier (Primary key)

The candidate key that has been selected as the unique characteristic for an entity type; underlined on an E-R diagram.

Multivalued attribute

An attribute that may take on more than one value for each entity instance (e.g., {Skills}).

Derived attribute

An attribute whose value can be computed from other data in the database (e.g., [Age] from Date of Birth).

Associative entity

An entity type that associates the instances of one or more entity types and contains attributes that are peculiar to the relationship.

Weak entity

An entity type whose existence depends on another entity type (shown with a double-lined rectangle).

Cardinality

The number of instances of entity B that can (or must) be associated with each instance of entity A.

Total specialization

A rule specifying that each entity instance of a supertype must be a member of at least one subtype (shown by a double line).

Disjoint rule

A rule specifying that if an entity instance is a member of one subtype, it cannot simultaneously be a member of any other subtype (shown by "d").

Triggering operation

An assertion or rule that governs the validity of data manipulation (Insert, Update, Delete) based on a specific condition.

Domain

The set of all data types and ranges of values that an attribute is allowed to assume.

Entity-Relationship Data Model (ER Model)

(E-R model) A detailed, logical representation of the entities, associations, and data elements for an organization or business area.

Entity-Relationship Diagram (ER Diagram)

(E-R diagram) A graphical representation of an E-R model.

Conceptual Data Model

A detailed model that captures the overall structure of organizational data and is independent of any database management system or other implementation considerations.

Entity Type

A collection of entities that share common properties or characteristics.

Entity Instance

A single occurrence of an entity type. Also known as an instance.

Required Attribute

An attribute that must have a value for every entity instance.

Optional Attribute

An attribute that may not have a value for every entity instance.

Composite Attribute

An attribute that has meaningful component parts.

Repeating Group

A set of two or more multivalued attributes that are logically related.

Relationship

An association between the instance of one or more entity types that is of interest to the organization.

Degree

The number of entity types that participate in a relationship.

Binary Relationship

A relationship between instances of two entity types. This is the most common type of relationship encountered in data modeling.

Ternary Relationship

A simultaneous relationship among instances of three entity types.

Unary Relationship

A relationship between instances of one entity type; also called recursive relationship.

Supertype

A generic entity type that has a relationship with one or more subtypes.

Subtype

A subgrouping of the entities in an entity type that is meaningful to the organization and that shares common attributes or relationships distinct from other subgroupings.

Overlap Rule

Specifies that an entity instance can simultaneously be a member of two (or more) subtypes.

Partial Specialization Rule

Specifies that an entity instance of the supertype does not have to belong to any subtype.

Business Rules

Specifications that preserve the integrity of the logical data model.