Entity Relationship Modeling Flashcards
What is an Entity Relationship Diagram?
It is a conceptual database as viewed by the end user
What are the main components of the ERD?
Entities
Attributes
Relationships
What are the 3 different ERD notations?
The Chen notation
The Crow’s Foot notation
The UML notation
What does the object of interest to the end user refer to?
It refers to the entity set and not to a single entity occurrence
What does the ERM correspond to?
It corresponds to a table in the relational environment
How is an entity represented in Chen. Crow’s Foot, and UML notations?
It is represented by a rectangle that contains the entity’s name
What are the characteristics of a required attribute?
Must have a value and cannot be left empty
What are the characteristics of a optional attribute?
Does not require a value and can be left empty
What are the characteristics of a domain?
Set of possible values for a given attribute
What are the characteristics of a identifier?
One or more attributes that uniquely identify each entity instance
What are the characteristics of a composite identifier?
Primary key composed of more than one attribute
What are the characteristics of a composite attribute?
Attribute that can be subdivided to yield additional attributes
What are the characteristics of a simple attribute?
Attribute that cannot be subdivided
What are the characteristics of a single-valued attribute?
Attribute that has only a single value
What are the characteristics of a multivalued attributes?
Attributes that have many values
What are the advantages of stored derived attributes?
Saves CPU processing cycles
Saves data access time
Data value is readily available
Can be used to keep track of historical data
What are the advantages of not stored derived attributes?
Saves storage space
Computation always yields current value
What are the disadvantages of stored derived attributes?
Requires constant maintenance to ensure derives value is current, especially if any values used in the calculation change
What are the disadvantages of not stored derived attributes?
Uses CPU processing cycles
Increases data access time
Adds coding complexity to queries
What is connectivity?
It describes the relationship classification
What is cardinality?
It expresses the minimum and maximum number of entity occurrences associated with one occurence of related entity
How is cardinality indicated in the ERD?
It is indicated by placing the appropriate numbers beside the entities, using the format (x,y)
What is existence dependence?
Entity exists in the database only when it is associated with another related entity occurrence
What is existence independence?
Entity exists apart from all of its related entities
Referred to as a strong entity or regular entity
What is a weak (non-identifying) relationship?
Primary key of the related entity does not contain a primary key component of the parent entity
What is a strong (identifying) relationship?
Primary key of the related entity contains a primary key component of the parent entity
What are the conditions of a weak entity?
1- Existence-dependent
2- Has a primary key that is partially or totally derived from parent entity in the relationship
How does a database designer determine whether an entity is weak?
Through business rules
What is optional participation?
One entity occurrence does not require a corresponding entity occurrence in a particular relationship
What is mandatory participation?
One entity occurrence requires a corresponding entity occurrence in a particular relationship
What does a relationship degree do?
It indicates the number of entities or participants associated with a relationship
What is a unary (recursive) relationship?
Association is maintained within a single entity
What is a binary relationship?
Two entities are associated
What is a ternary relationship?
Three entities are associated
What kind of relationship is found in a unary relationship?
A relationship can exist between occurrences of the same entity set
What is a pitfall when working with unary relationships?
One common pitfall when working with unary relationships is to confuse participation with referential integrity
What are associative entities?
Used to represent an M:N relationship between two or more entities
Has a 1:M relationship with the parent entities
Composed of the primary key attributes of each parent entity
May also contain additional attributes that play no role in connective process
