ER Diagram

An ER (Entity-Relationship) diagram is a visual blueprint of a database's structure - it shows the entities (things you want to store data about), their attributes, and how the entities relate to one another, before any table is actually created.

Core Components

• Entity - a real-world object or concept, like Customer or Order, usually drawn as a rectangle
• Attribute - a property of an entity, like name or email, drawn as an oval
• Relationship - how two entities are connected, like "Customer places Order", drawn as a diamond
• Primary Key - the attribute that uniquely identifies each entity instance, usually underlined

Types of Relationships

• One-to-One - one record in Table A relates to exactly one record in Table B
• One-to-Many - one record in Table A relates to many records in Table B (e.g. one Customer, many Orders)
• Many-to-Many - many records in Table A relate to many records in Table B (e.g. Students and Courses)

Why ER Diagrams Matter

Designing an ER diagram before writing any SQL helps catch structural problems early - like missing relationships or duplicated data - and makes it much easier to translate the design into actual CREATE TABLE statements.