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Questions and Answers
Questions and Answers
A strong entity type always has a primary key that uniquely distinguishes each occurrence of that entity.
A strong entity type always has a primary key that uniquely distinguishes each occurrence of that entity.
True (A)
A weak entity type can exist without a relationship with another entity.
A weak entity type can exist without a relationship with another entity.
False (B)
A table with a foreign key that can contain nulls is considered a strong entity.
A table with a foreign key that can contain nulls is considered a strong entity.
False (B)
The identifying owner is the weak entity type that depends on another entity.
The identifying owner is the weak entity type that depends on another entity.
A weak entity inherits its primary key entirely from its strong counterpart in the relationship.
A weak entity inherits its primary key entirely from its strong counterpart in the relationship.
EMPLOYEE is classified as a strong entity type and has Employee ID as its identifier.
EMPLOYEE is classified as a strong entity type and has Employee ID as its identifier.
A composite identifier is a primary key composed of only one attribute.
A composite identifier is a primary key composed of only one attribute.
Multivalued attributes in ER modeling can have many values associated with them.
Multivalued attributes in ER modeling can have many values associated with them.
An entity with a multivalued attribute should be implemented as is in a Relational Database Management System (RDBMS).
An entity with a multivalued attribute should be implemented as is in a Relational Database Management System (RDBMS).
Derived attributes in ER modeling must be physically stored within the database.
Derived attributes in ER modeling must be physically stored within the database.
Single-value attributes in ER modeling can have multiple values associated with them.
Single-value attributes in ER modeling can have multiple values associated with them.
In ER modeling, M:N relationships should not be implemented in the RDBMS.
In ER modeling, M:N relationships should not be implemented in the RDBMS.
A composite key is a primary key composed of only one attribute.
A composite key is a primary key composed of only one attribute.
Relationships between entities always operate in one direction only.
Relationships between entities always operate in one direction only.
A candidate key is a key that is unique but not minimal.
A candidate key is a key that is unique but not minimal.
Connectivity in ERD describes the relationship classification.
Connectivity in ERD describes the relationship classification.
An entity is said to be existence-independent if it can exist in the database without being associated with another related entity occurrence.
An entity is said to be existence-independent if it can exist in the database without being associated with another related entity occurrence.
Optional participation means that one entity occurrence requires a corresponding entity occurrence in a particular relationship.
Optional participation means that one entity occurrence requires a corresponding entity occurrence in a particular relationship.
A weak entity exists if the primary key of the related entity does not contain a component of the parent entity's primary key.
A weak entity exists if the primary key of the related entity does not contain a component of the parent entity's primary key.
Mandatory participation means that one entity occurrence does not require a corresponding entity occurrence in a particular relationship.
Mandatory participation means that one entity occurrence does not require a corresponding entity occurrence in a particular relationship.
Entity instances in an ER model can only be associated with one other entity instance.
Entity instances in an ER model can only be associated with one other entity instance.
An alternate key is always chosen as the primary key in ER modeling.
An alternate key is always chosen as the primary key in ER modeling.
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Study Notes
Study Notes
Entity Types
- Strong entity type: has a unique characteristic (identifier) that distinguishes each occurrence
- Weak entity type: existence depends on another entity type (existence-dependent) and has a primary key partially or totally derived from the parent entity
- Examples: EMPLOYEE (strong), DEPENDENT (weak)
Entity Characteristics
- Attributes: represented by ovals connected to entity rectangle with a line (Chen notation) or written in attribute box below entity rectangle (Crow’s Foot notation)
- Required attribute: must have a value
- Optional attribute: may be left empty
- Domain: set of possible values for an attribute
- Identifiers: one or more attributes that uniquely identify each entity instance
- Composite identifier: primary key composed of more than one attribute
Attribute Types
- Composite attribute: can be subdivided (e.g., ADDRESS, PHONE_NUMBER)
- Simple attribute: cannot be subdivided (e.g., AGE, SEX, MARITAL_STATUS)
- Single-value attribute: can have only a single value (e.g., SERIAL_NUMBER)
- Multivalued attributes: can have many values (e.g., DEGREE)
- Derived attribute: value may be calculated from other attributes (e.g., EMP_AGE)
Relationships
- M:N relationships and multivalued attributes should not be implemented directly in the RDBMS
- Create a new entity composed of original multivalued attribute’s components
- Identifiers (Primary Keys): one or more attributes that uniquely identify each entity instance
- Foreign Key: attribute in a table that references the primary key in another table
- Association between entities: participants are entities that participate in a relationship
- Connectivity: describes the relationship classification
- Cardinality: expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity
Existence Dependence
- Existence dependence: entity exists in database only when associated with another related entity occurrence
- Existence independence: entity can exist apart from one or more related entities
- Weak entity meets two conditions: existence-dependent, primary key partially or totally derived from parent entity in relationship
- Database designer determines whether an entity is weak based on business rules
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