Entity

How to read it

The rectangle stands for the whole set of things of one kind, not for one of them: “Order” means every order, and each individual order is an instance of the entity. Peter Chen’s 1976 paper, which introduced the entity-relationship diagram, drew each entity set as a rectangular box and each relationship as a diamond joined to the boxes by lines. A single-line rectangle is a regular, or strong, entity; a rectangle drawn with a double line is a Weak entity.

Also searched as: entity symbol, er diagram entity, strong entity.

Look-alikes

A Weak entity has a second rectangle inside the first. A rectangle on a flowchart is a process, a step that does something, while an entity is a thing the data describes. A UML class is also a rectangle, divided into compartments for its name, attributes and operations.

Differences by textbook

In Chen notation as Elmasri and Navathe draw it, attributes sit outside the rectangle in ovals. Silberschatz, Korth and Sudarshan’s Database System Concepts lists the attributes inside the entity rectangle instead and underlines the primary key. Crow’s foot diagrams also put the attributes inside the box, and Richard Barker’s notation draws entities as rectangles with rounded corners.