38 terms

Ch4 Entity Relationship Modeling

All terms
ER model
forms the basis of an ER diagram
represents conceptual database as viewed by end user
ERDs depict database's main components
- Entities
- Attributes
- Relationship
- 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
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
set of possible values for an attribute
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
-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
are entities that participate in a relationship
Describes the relationship classification
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
-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
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
Conflicting goals
design standards, processing speed, information requirements