ER Diagram (Entity-Relationship Diagram)

An ER (Entity-Relationship) diagram is a visual model of a database's design. It shows entities (things such as Customer or Order, which become tables), their attributes (which become columns) and the relationships between them.

Relationships are marked with their cardinality: one-to-one, one-to-many or many-to-many. A many-to-many relationship, such as students and courses, becomes a separate junction table with a composite key. ER diagrams are drawn before building tables to agree on the structure and spot design problems early.