Database Systems: ER Diagrams and Relationships
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Questions and Answers

What is the primary characteristic of a weak entity in entity-relationship modeling?

  • It has multiple attributes with high cardinality
  • It has a unique identifier
  • It participates in an identifying relationship with an owner entity (correct)
  • It has a recursive relationship with itself

What type of relationship exists between an entity and itself in entity-relationship modeling?

  • Unary relationship (correct)
  • N-ary relationship
  • Ternary relationship
  • Binary relationship

What is the term for the number of entities participating in a relationship in entity-relationship modeling?

  • Strength
  • Constraint
  • Degree (correct)
  • Cardinality

What is the relationship type that exists between three different entities in entity-relationship modeling?

<p>Ternary relationship (D)</p> Signup and view all the answers

What is the primary purpose of an identifying relationship in entity-relationship modeling?

<p>To identify a weak entity (D)</p> Signup and view all the answers

What is the term for the direction of operation in entity-relationship modeling?

<p>Bidirectional (B)</p> Signup and view all the answers

What is a characteristic of a weak entity?

<p>Its existence depends on another entity. (D)</p> Signup and view all the answers

What type of entity is used to implement M:N relationships between entities?

<p>Associative entity (D)</p> Signup and view all the answers

What is the purpose of an associative entity?

<p>To connect multiple entities in a M:N relationship (D)</p> Signup and view all the answers

What is a characteristic of a generalization entity?

<p>It encapsulates common characteristics of many entities (A)</p> Signup and view all the answers

What is an aggregation entity composed of?

<p>A set of entities (B)</p> Signup and view all the answers

What is the relationship between a class and a set of member classes in a generalization entity?

<p>Set-subset relationship (C)</p> Signup and view all the answers

What is the main difference between a stored attribute and a derived attribute?

<p>Derived attributes are calculated from other attributes, while stored attributes are manually entered by the user. (B)</p> Signup and view all the answers

In Chen notation, how is an attribute typically represented?

<p>As an oval containing its name and connected to the entity with a line. (D)</p> Signup and view all the answers

What is the main characteristic of a weak entity in entity-relationship modeling?

<p>It has a partial key and depends on another entity for its existence. (B)</p> Signup and view all the answers

What should be done when implementing a multivalued attribute in a relational database?

<p>Create a new entity for each of the original multivalued attributes’ components. (C)</p> Signup and view all the answers

What is the purpose of a partial key in a weak entity?

<p>To identify a unique instance of the weak entity. (C)</p> Signup and view all the answers

What is the main difference between a regular entity and a weak entity?

<p>A regular entity exists independently of other entities, while a weak entity exists only in relation to another entity. (B)</p> Signup and view all the answers

Flashcards

Single-valued Attribute

An attribute that can have only a single value. For example, a person's gender or a product's price.

Multivalued Attribute

An attribute that can have multiple values. For example, a student's list of courses or a product's list of features.

Stored Attribute

An attribute that must be provided by the user and stored in the database. For example, a customer's name or a product's description.

Derived Attribute

An attribute that is calculated from other attributes and doesn't need to be stored. For example, a customer's total purchase amount calculated from the quantity and price of items.

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Attribute Representation in ERD

A representation of an entity in an ERD. It is commonly used in the Chen notation and shows the name of the attribute in an oval.

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Regular Entity

An entity that exists independently of other entities. It has a primary key and is considered the main building block of the database.

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Weak Entity

An entity that depends on another entity for its existence. It doesn't have its own primary key but may have a partial key.

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Associative Entity

An entity that represents the association between two or more other entities. It is used to implement Many-to-Many relationships.

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Unary Relationship

A relationship type that involves only one entity. For example, an employee reporting to their manager, which is also an employee.

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Binary Relationship

A relationship type that involves two different entities. For example, a student taking a course.

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Ternary Relationship

A relationship type that involves three different entities. For example, an employee working on a project for a department.

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Identifying Relationship

A relationship between entities where the existence of the weak entity depends on the identifying entity.

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Partial Key

A key attribute that is used to identify a weak entity. It is composed of the primary key of the identifying entity and any additional attributes needed to uniquely identify the weak entity.

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Bidirectional Relationship

A relationship between entities that operates in both directions. For example, a student taking a course also means the course has a student enrolled.

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Relationship Cardinality

A relationship between entities that is expressed with a degree of involvement between the entities. This can indicate a relationship of one-to-one, one-to-many, or many-to-many.

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Relationship Constraint

A type of relationship that shows how many instances of one entity can be related to another. For example, a one-to-one relationship means one instance of an entity is related to one instance of another entity. This type of relationship will have a maximum number of instances of each entity within the relationship.

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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.

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Test your knowledge of Enhanced Entity-Relationship Diagrams (EER/E2R) and relationships between entities. Learn about specialization, generalization, and association between entities.

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