How to Create an Entity-Relationship Diagram for an Auto Service Business

Entity-relationship (ER) diagrams demonstrate relationships between the persons, places and things in an enterprise. An auto service business Crow's Foot entity-relationship diagram illustrates how the components of the enterprise such as customers, vehicles, mechanics and invoices work together. Create the entity-relationship diagram for an auto service business as basic or as detailed as needed.

The Steps

Define the entities (person, places or things) of an auto service business. Entities include customers, vehicles, mechanics, customer service assistants, salaries, invoices and customer records.

Define the relationships between the entities. Identify the relationships as verbs in the form “entity – relationship – entity.” Relationships include “mechanics service vehicles,” “customer service assistants manage customer records,” “mechanics receive salaries,” “customer service assistants receive salaries,” “customer service assistants help customers” and “customers pay invoices.”

Draw the entity-relationship diagram. Write the name of each entity inside its own rectangle. Draw a line between entities that have relationships. Write the name of each relationship above its corresponding line.

Define the cardinality (maximum number of instances, either “many” or “one) and modality (minimum number of instances, either “one” or “zero”) of the relationships. For example, one mechanic services many vehicles, one customer service assistant manages many customer records, one mechanic receives one salary, one customer service assistant helps many customers and one customer pays one invoice.

Write the cardinality of the relationships on the outer ends of the relationship lines next to each entity. Denote a cardinality of “many” by drawing on the relationship line a crow's foot with three toes (similar to the letter E) touching the entity. Denote a cardinality of “one” by drawing a straight line perpendicular to the relationship line immediately next to the entity.

Write the modality of the relationships next to the cardinality. Denote a modality of one by drawing a straight line perpendicular to the relationship line (as in denoting cardinality). Denote a modality of zero by drawing a circle.


  • Define as many entities and relationships as needed before drawing the entity-relationship diagram.