Crow's Foot Notation Explained: The 4 Cardinality Symbols (Visio ER Diagram)
One and only one, one or many, zero or many, zero or one. Learn the four crow's foot symbols and how to read them out loud, drawn on a small HR data model in Visio.
Step by step
- Crow's foot symbols. Crow's foot notation has just four symbols on the end of a line. Learn them once, and you can read any ER diagram. Let's draw them on a small HR model in Visio.
- Crow's Foot stencil. I'm using the Crow's Foot Database Notation stencil.
- The entities. First, a department entity, with its key, a name and a budget. Then employee. Each employee row keeps the ID of the department they work in.
- 1 + 2 - One / one or many. Connect them. On the department end, two short bars mean one and only one. On the employee end, a bar and a crow's foot mean one or many.
- Read it out loud. Read it out loud. Each department has one or many employees. Each employee works in one and only one department.
- Zero or one. Some employees get a parking permit, and nobody gets two. Add a permit entity. Connect it. A circle and a bar mean zero or one. An employee has zero or one permit.
- Zero or many. Last, leave requests. A new starter may have none yet, and others have lots. Connect it. A circle and a crow's foot mean zero or many. An employee has zero or many leave requests.
- Done. Click away to see all four symbols on one diagram.
- The 4 symbols. So, the symbol nearest the box is the maximum, a bar for one or a crow's foot for many. The one further out is the minimum, a circle for zero or a bar for one.
