Free ER Diagram Maker — Draw Entity-Relationship Diagrams Online

Quick Answer

What is an ER diagram and what are the three types of relationships?

An Entity-Relationship (ER) diagram maps the data model of a database: entities (tables), attributes (columns), and relationships between them. The three relationship types are: (1) One-to-One (1:1) — each record in Table A relates to exactly one in Table B (e.g. User ↔ Profile); (2) One-to-Many (1:N) — one record in A maps to many in B (e.g. Author → Books); (3) Many-to-Many (M:N) — multiple records on both sides (e.g. Students ↔ Courses), requiring a junction table.