DATABASES

Entity-Relationship Model (ERM)

- Forms the basis of an Entity-Relationship Diagram, which represents the conceptual database as viewed by the end user.

### Attributes

- Single-Valued Attribute: An attribute that can have only a single value.

- Multivalued Attribute: An attribute that can have many values.

- Derived Attribute: An attribute whose value is calculated from other attributes.

- Required Attribute: An attribute that must have a value.

- Optional Attribute: An attribute that does not require a value.

- Domain: A set of possible values for a given attribute.

- Identifiers (Primary Key - PK): One or more attributes that uniquely identify each entity instance.

- Composite Identifier: A primary key composed of more than one attribute.

- Composite Attribute: An attribute that can be further subdivided to yield additional attributes.

- Simple Attribute: An attribute that cannot be subdivided.

### Relationships

- Participants: Entities that participate in a relationship.

- Connectivity: Used to describe the relationship classification.

- Cardinality: Expresses the minimum and maximum number of entity occurrences associated with one occurrence of the related entity.

- Existence Dependence: A property of an entity whose existence depends on one or more other entities.

- Existence Independence: A property of an entity that can exist apart from related entities.

- Strong Entity: An entity that is existence-independent and can exist apart from all of its related entities.

- Weak Entity: Displays existence dependence and inherits the primary key from another entity.

- Weak Relationship (Non-Identifying): Does not contain a primary key component of the parent entity.

- Strong Relationship (Identifying): Exists when the primary key of the related entity contains a primary key component.

- Optional Participation: One entity occurrence does not require a corresponding entity occurrence in a relationship.

- Mandatory Participation: One entity occurrence requires a corresponding entity occurrence in a relationship.

### Types of Relationships

- Unary Relationship: Describes association within the same entity.

- Binary Relationship: Exists when two entities are associated.

- Ternary Relationship: Exists when three entities are associated.

- Recursive Relationship: A relationship that exists between occurrences of the same entity.

- Relationship Degree: Indicates the number of entities or participants associated with a relationship.

### Additional Concepts

- Relational Schema: The organization of a relational database.

- Iterative Process: A process based on repetition of steps and procedures.

This document highlights key terms and concepts from the Entity-Relationship Model (ERM) to help with understanding and memorization.