Create Your First Entity in an ERD

What is an Entity?

In ER modeling, an entity represents a real-world object or concept that you want to store data about. In relational database terms, an entity maps directly to a table. Each entity has a name and a set of attributes (columns) that describe its properties.

Anatomy of an Entity

A well-defined entity includes:

Component	Purpose	Example
Entity name	Identifies the table	users
Primary key	Uniquely identifies each row	id SERIAL
Attributes	Data stored about the entity	name VARCHAR(255)
Data types	Define what kind of data each column holds	INT, TEXT, BOOLEAN
Constraints	Rules like NOT NULL, UNIQUE	email VARCHAR(320) NOT NULL UNIQUE

Step-by-Step: Creating Your First Entity

1. Choose a clear name: Use a plural noun in snake_case (e.g., users, products, order_items). The name should describe what collection of things the table stores.

2. Define the primary key: Almost every entity needs a primary key column. The most common patterns are:

   • id SERIAL — auto-incrementing integer (PostgreSQL)
   • id INT AUTO_INCREMENT — auto-incrementing integer (MySQL)
   • id UUID — universally unique identifier

3. Add attributes: Think about what data you need to store. Each attribute becomes a column with a data type. Start with the essential columns and add more later.

4. Set constraints: Mark columns as NOT NULL if they are required, UNIQUE if values must not repeat, and set DEFAULT values where appropriate.

Example: A Simple Users Entity

CREATE TABLE users (
  id SERIAL PRIMARY KEY,
  username VARCHAR(255) NOT NULL UNIQUE,
  email VARCHAR(320) NOT NULL UNIQUE,
  is_active BOOLEAN NOT NULL DEFAULT true,
  created_at TIMESTAMP NOT NULL DEFAULT NOW()
);

In the ERD editor, this entity appears as a card with the table name in the header and each column listed below, color-coded by key type: gold for the primary key (id) and the default color for regular columns.