Chapter 4

Domain

Set of possible values for a given attribute

Optional participation

one entity occurrence does not require a corresponding entity occurrence in a particular relationship

composite identifier

primary key composed of more than one attribute

Relationship Degrees

unary, binary, ternary

Disadvantages of Storing Derived Attributes

- Requires constant maintenance to ensure derived value is current, especially if any values used in the calculation change.

Advantages of Storing Derived Attributes

- Saves CPU processing cycles - Saves data access time - Data value is readily available - Can be used to keep track of historical data

Advantages of not storing derived attributes

- Saves storage space - Computation always yields current value

Disadvantages of not storing derived attributes

- Uses CPU processing cycles - Increases data access time - Adds coding complexity to queries

Attribute

Characteristics of entities

Relationship

Association between entities that always operate in both directions

Unary relationship

Association is maintained within a single entity

composite attribute

Attribute that can be subdivided to yield additional attributes

Simple attribute

Attribute that cannot be subdivided

single-valued attribute

Attribute that has only a single value

Derived attribute

Attribute whose value is calculated from other attributes. (Derived from algorithm)

Multivalued attributes

Attributes that have many values and require creating: - Several new attributes, one for each component of the original multivalued attribute - A new entity composed of the original multivalued attribute's components

ERD depicts the

Conceptual database as viewed by end user. Database's main components (entities, attributes, relationships)

Developing an ER Diagram

Create a detailed narrative of the organization's description of operations. ID business rules based on descriptions ID main entities and relationships from business rules Develop initial ERD ID attributes & PK's for entities Revise and review ERD

Connectivity

Describes the relationship classification

Optional attribute

Does not require a value, can be left empty

Participants

Entities that participate in a relationship

Existence independence

Entity exists apart from all of its related entities Referred to as a strong entity or regular entity

Existence dependence

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

Weak Entity

Existence dependent Has a PK that is partially or totally derived from parent entity in the relationship. For example, a DEPENDENT requires the existence of an EMPLOYEE.

Cardinality

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

Required attribute

Must have a value, cannot be left empty

Mandatory participation

One entity occurrence requires a corresponding entity occurrence in a particular relationship

Identifiers

One or more attributes that uniquely identify each entity instance.

Relationship types

Participants Connectivity Cardinality

strong (identifying) relationship

Primary key of the related entity contains a primary key component of the parent entity

Weak (non-identifying) relationship

Primary key of the related entity does not contain a primary key component of the parent entity

Recursive relationship

Relationship exists between occurrences of the same entity set

Associative (Composite) Entities

Represents M:N relationship between entities Is a 1:M relationship with the parent entities Has PK attributes of each parent

Attribute terms

Required attribute Optional attribute Domain Identifiers Composite Identifier Composite attribute Simple attribute Single-valued attribute Multivalued attributes Derived attribute

Ternary relationship

Three entities are associated

Binary relationship

Two entities are associated

Relationship Degree

indicates the number of entities or participants associated with a relationship