Explain key components of an ER diagram.

An ER diagram, or Entity-Relationship diagram, is a visual representation of the entities and relationships within a database. Key components of an ER diagram include:

1. Entities: Entities are objects or concepts that exist independently and have attributes that describe them. In an ER diagram, entities are represented as rectangles.

2. Attributes: Attributes are properties or characteristics of an entity. They provide more detailed information about the entity. Attributes are represented as ovals connected to the entity they belong to.

3. Relationships: Relationships describe how entities are related to each other. They show how data is shared between entities. Relationships are represented as diamond shapes connecting two entities.

4. Cardinality: Cardinality describes the number of instances of one entity that can be associated with another entity. It is represented using symbols such as "1" for one-to-one relationships, "M" for many-to-one relationships, and "N" for many-to-many relationships.

5. Primary key: A primary key is a unique identifier for each record in a database table. It is used to uniquely identify each record and is typically represented in an ER diagram with an underline.

6. Foreign key: A foreign key is a field in a database table that links to the primary key of another table. It is used to establish relationships between tables. Foreign keys are represented in an ER diagram with a dashed underline.

7. Weak entities: Weak entities are entities that do not have a primary key of their own and rely on a strong entity for identification. They are represented in an ER diagram with a double rectangle.

Overall, an ER diagram helps to visualize the structure of a database, including the entities, attributes, relationships, and constraints that exist within it. It is a valuable tool for database designers and developers to understand and communicate the database schema.

Key Components of an Entity-Relationship (ER) Diagram:

1. Entities:
- Represents real-world objects or concepts that have distinct and identifiable characteristics.
- Depicted as rectangles with their names inside.

2. Attributes:
- Properties or characteristics that describe entities.
- Represented as ovals connected to entities.
- Example: Customer ID, Customer Name

3. Relationships:
- Connections between entities that represent associations or interactions.
- Depicted as lines connecting entities with specific symbols and labels.

4. Relationship Types:

One-to-One:
- Each entity in one set is related to only one entity in the other set.
- Represented as a solid line with a "1" at both ends.

One-to-Many:
- Each entity in one set can be related to multiple entities in the other set.
- Represented as a line with a "1" at one end and a "N" (or asterisk) at the other end.

Many-to-Many:
- Entities in both sets can have multiple relationships with each other.
- Represented as a line with a "N" at both ends.

5. Cardinality:
- Represents the minimum and maximum number of entities that can participate in a relationship.
- Indicated by numbers or symbols (e.g., 1..-, 0..1) on the relationship line.

6. Primary Key:
- A unique identifier that distinguishes one entity from all others within the same entity set.
- Depicted as an underlined attribute.

7. Foreign Key:
- An attribute in one entity that references the primary key of another entity.
- Used to establish relationships between entities.

8. Weak Entity:
- An entity that does not have its own primary key and is dependent on a strong entity.
- Depicted as a rectangle with double lines and is connected to a strong entity by a line with a double diamond.

9. Composite Key:
- A primary key that consists of two or more attributes used together to uniquely identify an entity.
- Represented as a rectangle with attributes connected by a dashed line.