This SQL ER (Entity Relationship) Diagram depicts the table's structure and the relationships between logically related tables. The database structure is an ER diagram in ER modeling (ERD). An ER diagram helps you understand the overall design of the database. The tables, their keys, and their relationships become easier to map. To create an ER diagram, the tables must be logically related to one another with foreign key constraints. Entity Relationship Diagram, also known as ERD, is a diagram that depicts the relationship of entity sets stored in a database.