Questions and Answers
What is the primary characteristic of a weak entity in entity-relationship modeling?
It participates in an identifying relationship with an owner entity
What type of relationship exists between an entity and itself in entity-relationship modeling?
Unary relationship
What is the term for the number of entities participating in a relationship in entity-relationship modeling?
Degree
What is the relationship type that exists between three different entities in entity-relationship modeling?
Signup and view all the answers
What is the primary purpose of an identifying relationship in entity-relationship modeling?
Signup and view all the answers
What is the term for the direction of operation in entity-relationship modeling?
Signup and view all the answers
What is a characteristic of a weak entity?
Signup and view all the answers
What type of entity is used to implement M:N relationships between entities?
Signup and view all the answers
What is the purpose of an associative entity?
Signup and view all the answers
What is a characteristic of a generalization entity?
Signup and view all the answers
What is an aggregation entity composed of?
Signup and view all the answers
What is the relationship between a class and a set of member classes in a generalization entity?
Signup and view all the answers
What is the main difference between a stored attribute and a derived attribute?
Signup and view all the answers
In Chen notation, how is an attribute typically represented?
Signup and view all the answers
What is the main characteristic of a weak entity in entity-relationship modeling?
Signup and view all the answers
What should be done when implementing a multivalued attribute in a relational database?
Signup and view all the answers
What is the purpose of a partial key in a weak entity?
Signup and view all the answers
What is the main difference between a regular entity and a weak entity?
Signup and view all the answers
Study Notes
Attributes
- Single-value attributes can have only a single value.
- Multivalued attributes can have many values.
- Stored attributes must be entered by the user and stored in the database.
- Derived attributes are calculated from other attributes and are not required to be stored in the database.
- M:N relationships and multivalued attributes should not be implemented in relational tables.
- Create several new attributes for each of the original multivalued attributes' components.
- Create a new entity composed of original multivalued attributes' components.
Attributes Representation in ERD
- Chen notation: attributes are represented by an oval containing its name and connected to the entity with a line.
- Crow's Foot notation: attributes are written in an attribute box below the entity rectangle.
Types of Entities
Regular/Strong Entities
- Existence does not depend on any other entity.
- Has a key attribute.
- Examples: Employees, Departments, Projects, and Students entities.
Weak/Subordinate Entities
- Existence depends on another entity.
- Has no key attributes of its own, but may have a Partial key.
- Weak entities are identified by the combination of their Partial key (if exists) and Primary key of its identifying entity.
- Examples: Dependent and Invoice entities.
Associative/Composite/Bridge Entities
- Existence depends on two or more entities.
- Used to implement M:N relationship between entities.
- Composed of all key attributes of all connected entities.
- May contain additional attributes that play no role in the connective process.
- Has no key attributes of its own, but may have a Partial key.
- Associative entities are identified by the combination of their Partial key (if exists) and all Primary keys of their identifying entities.
- Examples: student grades depend on the student and the course.
Generalization
- Encapsulates common characteristics of many subordinate entities.
- Defines a set-subset relationship between a class and a set of member classes.
- Used with Enhanced Entity Relationship Diagram (EER/E2R).
- Examples: a four-wheeler is a type of vehicle, and a truck is a type of four-wheeler.
Aggregation
- Consists of or an aggregation of other entities.
- Defines a new entity from a set of entities which are identified as components of the root entity.
- Used with Enhanced Entity Relationship Diagram (EER/E2R).
- Examples: a car consists of engine, chassis, gear box, etc.
Relationships
- Association between entities.
- Participants are entities that participate in a relationship.
- Relationships between entities always operate in both directions.
- More than one relationship type can exist with the same participating entity types.
- The relationship type has three main characteristics: Degree, Strength, and Cardinality/Constraints.
- A weak entity must participate in an identifying relationship type with an owner or identifying entity type.
Relationships Degree
- Unary/Recursive: Relationship between an entity and itself.
- Binary: Relationship between two different entities.
- Ternary: Relationship between three different entities.
- Nary: Relationship between N different entities.
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.
Description
Test your knowledge of Enhanced Entity-Relationship Diagrams (EER/E2R) and relationships between entities. Learn about specialization, generalization, and association between entities.