Untitled Quiz

Study Notes

Explanations of terms or facts.
Terms represent words or phrases with specific meanings.
Facts denote an association between two or more terms.
Guidelines for creating good definitions:
- Gather definitions during systems requirement analysis.
- Accompany definitions with diagrams for visual representation.
- Iteratively refine definitions through collaboration.
- Achieve consensus on definitions.

Entities:
- Entity instance: A specific person, place, object, event, or concept (often corresponds to a row in a table).
- Entity type: A collection of entities (often corresponds to a table).
Relationships:
- Relationship instance: Links between entities (corresponds to primary key-foreign key equivalencies in related tables).
- Relationship type: Categories of relationships linking entity types.
Attribute: A property or characteristic of an entity or relationship type (often corresponds to a field in a table).

Should be:
- An object with many instances in the database.
- An object composed of multiple attributes.
- An object being modeled.
Should not be:
- A user of the database system.
- An output of the database system (e.g., a report).

Required vs. Optional: Whether an attribute must be present for each entity instance.
Simple vs. Composite: Whether an attribute is a single value or broken down into components.
Single-Valued vs. Multivalued: Whether an attribute can hold one value or multiple values for each instance.
Stored vs. Derived: Whether an attribute is physically stored or calculated from other attributes.
Identifier Attributes (Keys): Attributes that uniquely identify individual instances of an entity type.

Unchanging in value.
Not null.
No intelligent identifiers (e.g., containing locations or people that might change).
Substitute simple keys for complex composite keys.

Relationship type: Modeled as lines between entity types. Represents the category of association.
Relationship instance: Instances of the relationship type linking specific entity instances.
Relationships can have attributes that describe features of the association between entities.
Two entities can have multiple relationship types between them.
Associative Entity: Combines elements of relationships and entities, representing an association with attributes.

One-to-One: Each entity on both sides has exactly one related entity.
One-to-Many: One entity can have multiple related entities on the other side, but each entity on the other side can only have one related entity.
Many-to-Many: Both entities can have multiple related entities on the other side.

Define the number of instances of one entity that can or must be associated with each instance of another entity.
Minimum Cardinality:
- Zero: Optional relationship.
- One or more: Mandatory relationship.
Maximum Cardinality: The maximum number of instances allowed in the relationship.

Strong entity: Exists independently. Has its own unique identifier.
Weak entity: Dependent on a strong entity (identifying owner). Cannot exist without the strong entity. Does not have a unique identifier (only a partial identifier).
Identifying Relationship: Links strong entities to weak entities.