This ER Diagram represents the relationship between entities in a table for the Hospital Management System. ER diagrams define database logical structure. An ER Diagram represents the relationship between two database tables. Entity Relationship Diagram (E-R diagram) Entity is a system object; generally, we refer to entities as database tables; the e-r diagram depicts the relationship between each database table. The E-R diagram represents an entity with attributes, which are properties of the Entity. If we assume the Entity is a database table, all of the table's columns are treated as attributes.