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Questions and Answers
What characterizes a weak entity?
What characterizes a weak entity?
- It is represented by a single-lined box.
- It can exist independently of a strong entity.
- It has its own unique identifier.
- It relies on a strong entity and has a partial identifier. (correct)
What is the purpose of an identifying relationship?
What is the purpose of an identifying relationship?
- To replace strong entities.
- To create new entities.
- To provide unique identifiers for entities.
- To connect strong entities with weak entities. (correct)
When should a relationship with attributes be represented as an associative entity?
When should a relationship with attributes be represented as an associative entity?
- When all relationships involved are one-to-many.
- When it can participate in other relationships. (correct)
- When it is a one-to-one relationship.
- When it has a partial identifier.
How is a partial identifier represented in a weak entity?
How is a partial identifier represented in a weak entity?
What indicates that an associative entity has additional meaning independent of the other entities?
What indicates that an associative entity has additional meaning independent of the other entities?
Which of the following correctly describes strong entities?
Which of the following correctly describes strong entities?
What feature sets associative entities apart from standard relationships?
What feature sets associative entities apart from standard relationships?
What is indicated by a double-lined box in an entity model?
What is indicated by a double-lined box in an entity model?
What is an appropriate characteristic of an entity in a database?
What is an appropriate characteristic of an entity in a database?
Which type of attribute indicates that its value cannot be empty?
Which type of attribute indicates that its value cannot be empty?
What defines a composite attribute?
What defines a composite attribute?
Which of the following statements about identifiers is true?
Which of the following statements about identifiers is true?
In the context of database attributes, what does the term 'Derived Attribute' refer to?
In the context of database attributes, what does the term 'Derived Attribute' refer to?
Which of the following best describes a candidate identifier?
Which of the following best describes a candidate identifier?
What is a common type of attribute classification?
What is a common type of attribute classification?
Which characteristic is essential for an identifier in a database?
Which characteristic is essential for an identifier in a database?
What distinguishes a simple attribute from a composite attribute?
What distinguishes a simple attribute from a composite attribute?
What is an associative entity?
What is an associative entity?
What degree of relationship involves three different entity types?
What degree of relationship involves three different entity types?
In a one-to-many relationship, how many related entities can exist for the entity on the 'one' side?
In a one-to-many relationship, how many related entities can exist for the entity on the 'one' side?
What type of cardinality constraint indicates that an entity must have at least one relation?
What type of cardinality constraint indicates that an entity must have at least one relation?
What is the main purpose of business rules in an organization?
What is the main purpose of business rules in an organization?
Which of the following describes a unary relationship?
Which of the following describes a unary relationship?
What does an entity instance correspond to in a database?
What does an entity instance correspond to in a database?
What happens in a many-to-many relationship?
What happens in a many-to-many relationship?
Which of the following is NOT a type of relationship?
Which of the following is NOT a type of relationship?
What is the minimum cardinality of a relationship when it is optional?
What is the minimum cardinality of a relationship when it is optional?
What is an associative entity used for in database modeling?
What is an associative entity used for in database modeling?
What defines the cardinality of a relationship in an E-R diagram?
What defines the cardinality of a relationship in an E-R diagram?
Which type of relationship involves two entity types only?
Which type of relationship involves two entity types only?
What does maximum cardinality determine in a relationship?
What does maximum cardinality determine in a relationship?
Which of the following best describes an attribute in an E-R model?
Which of the following best describes an attribute in an E-R model?
Which scenario best describes a multivalued attribute represented as a relationship?
Which scenario best describes a multivalued attribute represented as a relationship?
What type of notation is commonly used to represent relationships in E-R diagrams?
What type of notation is commonly used to represent relationships in E-R diagrams?
What is the relationship instance typically associated with in a relational database?
What is the relationship instance typically associated with in a relational database?
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Study Notes
Modeling Data in Organizations
- Data modeling is essential for defining business structures and controlling behaviors through business rules.
- Business rules are statements crafted in terms familiar to users, automated using Database Management System (DBMS) software.
E-R Model Constructs
- Entities: Defined by instance (single occurrence, e.g., a person) and type (collection of instances, e.g., table).
- Relationships: Links between entities categorized as instances (specific connections) and types (general associations).
- Attributes: Characteristics of entities or relationships, often reflected as fields in tables.
Characteristics of Entities
- Should represent objects with multiple instances, multiple attributes, and are the focus of modeling.
- Not to be confused with database users or reports.
Attribute Classifications
- Required vs. Optional: Specifies necessity.
- Simple vs. Composite: Simple is single-valued; composite consists of multiple parts.
- Single-Valued vs. Multivalued: Single allows one value; multivalued allows several.
- Stored vs. Derived: Stored is kept in the database; derived is calculated from other attributes.
- Identifier Attributes: Uniquely identify an entity instance.
Identifiers (Keys)
- Unique attributes or combinations that distinguish entity instances.
- Simple vs. Composite Identifiers: Simple consists of one attribute; composite consists of multiple attributes.
- Candidate identifiers meet identification criteria.
Attributes and Relationships
- Relationship types are represented by lines, while instances connect specific entities.
- Relationships can have attributes describing associations.
Degrees of Relationships
- Unary: Involves one entity type.
- Binary: Involves two entity types.
- Ternary: Involves three entity types.
Cardinality of Relationships
- One-to-One: One entity to one related entity.
- One-to-Many: One entity to multiple related entities on one side.
- Many-to-Many: Many entities on both sides can relate.
Cardinality Constraints
- Define how many instances of one entity relate to another.
- Minimum cardinality dictates optionality (zero is optional; one or more is mandatory).
- Maximum cardinality indicates the upper limit of associations.
Identifying Relationships
- Strong Entities: Exist independently, own unique identifiers.
- Weak Entities: Depend on strong entities, lack unique identifiers, defined by partial identifiers.
- Identifying Relationships: Connect strong and weak entities, indicating dependency.
Associative Entities
- Combine attributes of a relationship while also functioning as an entity.
- Selected for relationships with many cardinalities, independent meaning, unique identifiers, or when participating in other relationships.
Time-Dependent Data
- Time stamps are utilized to capture historical data accurately, aiding in the contextual analysis of time-sensitive information.
Visual Representations
- E-R diagrams effectively visualize entities, relationships, attributes, cardinalities, and identifying relationships for complex scenarios.
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