Entity-Relationship Diagram

An ER diagram, or Entity Relationship diagram, provides a visual depiction of the logical structure of a database.

• It illustrates the constraints and relationships between various components.
• An ER diagram typically consists of three primary components: Entity Sets, Attributes, and Relationship Sets.

Stud_id, Col_id is the primary key.
Student, College are entity sets of which (Stud_id, Stud name, Stud addr) and (Col_id, Col name) are attributes of respectively.
Study In is the relationship between Student and College entities.

Entity Set

It is a set of the same type of entities.

1. Strong Entity Set: In a strong entity set, the primary key exists. A primary key in an entity set is represented by underlining it.
2. Weak Entity Set: No primary key exists. It contains a partial key called discriminator.

Relationship

An association among entities.
It can be unary, binary, ternary, or n-ary.

Cardinality Constraint

Maximum number of relationship instances an entity set can take part in.
It can be:

• One-to-one
• One-to-many
• Many-to-one
• Many-to-many

Attributes

Attributes are the descriptive characteristics possessed by each entity within an Entity Set.

Types of Attributes:

1. Simple Attribute: A single atomic value is associated with each instance of the attribute. For example, "Age" or "Name".
2. Composite Attribute: It comprises multiple simple attributes. For instance, an "Address" attribute may include "Street", "City", and "Zip Code" components.
3. Derived Attribute: This attribute's value is derived or calculated from other attributes. For example, "Age" can be derived from the "Date of Birth".
4. Single-valued Attribute: It holds a single value for each instance of the entity. For instance, "Height" or "Weight".
5. Multi-valued Attribute: It can hold multiple values for each instance of the entity. For example, "Phone Numbers" or "Skills".
6. Key Attribute: It uniquely identifies each entity within an entity set. For example, "ID" or "Employee Number".

Extended ER Features

Extended Entity-Relationship (EER) features in database management systems (DBMS) extend the capabilities of traditional Entity-Relationship (ER) models.
These features include:

1. Generalization: Aggregates common attributes and relationships from multiple entities to form a supertype.
2. Specialization: The process of defining subtypes based on a subset of attributes and relationships from a supertype.
3. Aggregation: EER models allow entities and relationships to be aggregated into higher-level constructs known as aggregates.
   This enables the modeling of complex relationships between groups of entities or relationships.