Here's an entity relation diagram for KEGG. An Entity–relationship model (ER model) uses a diagram known as an Entity Relationship Diagram to illustrate the structure of a database (ER Diagram). An ER model is a database architecture or blueprint that may subsequently be implemented as a database. The entity set and relationship set are the two fundamental components of the E-R model. An ER diagram depicts the connection between entity sets. An entity set is a collection of comparable entities, each of which can have characteristics.