ER Diagrams and Chen Notation

Study Notes

ER Diagrams (ERD) represent the relationships between different entities in a database.
ERDs are used to model data and develop databases.
ERDs are commonly used in database design and development.

Entities: Entities are things or concepts in the database that can have data stored about them.
- Entities are typically represented by rectangles.
- Examples of entities include customers, products, or orders.
Relationships: Relationships represent how entities interact with each other.
- Relationships are typically represented by diamonds or labels on connecting lines.
- Examples of relationships include "customer orders product."
Attributes: Attributes are characteristics of entities.
- Attributes are typically represented by ovals or circles.
- Examples of attributes include customer name, product price, or order date.

Cardinality defines the numerical relationship between entities.
There are three main types of cardinality:
- One-to-one: One entity is related to exactly one other entity.
- One-to-many: One entity is related to multiple other entities.
- Many-to-many: Multiple entities are related to multiple other entities.

ER models and data models are typically drawn at three levels of detail:
- Conceptual Data Model: The highest level view with the least detail.
- Logical Data Model: Contains more detail than the conceptual model and defines operational and transactional entities.
- Physical Data Model: Developed from the logical model and represents the physical implementation of the database, including specific data types and structures.

Entities can be categorized as follows:
- Strong Entity: Can be defined solely by its own attributes.
- Weak Entity: Requires an identifying relationship with another entity to be defined.
- Associative Entity: Represents an association between entities or elements within an entity set.

Super Key: A set of attributes that uniquely defines an entity.
Candidate Key: A minimal super key with the least possible number of attributes.
Primary Key: A candidate key chosen to uniquely identify an entity.
Foreign Key: An attribute that identifies the relationship between entities.

Identify all relevant entities and relationships for your database.
Watch for redundant entities or relationships.
Ensure all entities and relationships are labeled clearly.
Translate between relational tables and ER diagrams if necessary.
Ensure the diagram supports all the data needed to be stored.
Multiple valid approaches exist, as long as the ERD provides the necessary information for its purpose.