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Entity Relationship Diagram (ERD) 101

What ERD is, Types of ERDs: Conceptual, Logical, Physical

Hi. In this text, I will write about Entity Relationship Diagrams that some of us know as ERDs.

Photo by Leif Christoph Gottwald on Unsplash

ERDs are a graphical representation of database designs. There are 3 types of ERD.

  • Conceptual Design
  • Logical Design
  • Physical Design

ERDs help us to figure out a database schema and logic. Also, they help us to document our databases.

Crow’s Foot

Crow’s Foot is a notation that we use in ERDs. In this text, I will use it without cardinality symbols to keep it simple. However, it could be helpful to know that there are symbols that depict the cardinality level.

Crow’s Foot basically indicates that if there are 3 lines on the relationship, it says the 3 line’s side is multi (many)-type relationship (eg. category can have more than 1 product). If there is just 1 line on the relationship, it is a single type relationship.

Conceptual Design

Conceptual design presents a high-level picture of the system with a focus on the business objects involved in the system. At this level, database tables are not designed.

Conceptual Design

Logical Design

In Logical Design, business objects or entities have attributes (columns). These attributes begin to detail the information that makes up a single record (or row) within an eventual database table. However, there is still no defined RDBMS specific types for attributes. This level of design provides a point of what will be in a single record to business stakeholders.

Logical Design

Physical Design

This level of design is the level where we define RDBMS specific data types for each attribute. Also, we need to pay attention while defining data types because the constructs, conventions or restrictions of a specific RDBMS can vary.

Physical Design (data types for MySQL)


Hopefully, this text helps you to understand what ERD is and what ERD types are. Sometimes we think that these kinds of theoretical stuff are not important. However, once you understand their purpose and what they solve and how they help us to develop projects, you will put them into your toolbox.




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Lifelong learner & Freelancer. I use technology that helps me. I’m currently working as a Business Intelligence & Backend Developer.