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 uses 4 symbols on the ends of relationship lines in an ER diagram. Once you know them, you can read most ER diagrams. Here they are drawn on a small HR data model in Visio.
- Crow's Foot stencil. The shapes come from Visio's Crow's Foot Database Notation stencil, shown in the Shapes pane.
- The entities. First, add a Department entity with 3 attributes: DepartmentID (its key), Name and Budget. Then add an Employee entity with EmployeeID, Name and DepartmentID. The DepartmentID in each employee row shows which department that person works in.
- 1 + 2 - One / one or many. Connect Department to Employee with a Relationship connector. 2 short bars on the Department end mean one and only one. A bar and a crow's foot on the Employee end mean one or many.
- Read it out loud. Read it out loud in both directions: each department has one or many employees, and each employee works in one and only one department.
- Zero or one. Some employees get a parking permit, and nobody gets 2. Add a ParkingPermit entity with PermitID, EmployeeID and CarPark. Connect Employee to ParkingPermit. A circle and a bar on the ParkingPermit end mean zero or one, so an employee has zero or one permit.
- Zero or many. Last, leave requests. A new starter may not have any yet, and others have lots. Add a LeaveRequest entity with LeaveID, EmployeeID and StartDate. Connect Employee to LeaveRequest. A circle and a crow's foot on the LeaveRequest end mean zero or many, so an employee has zero or many leave requests.
- Done. Click an empty part of the page so nothing is selected. Now you can see all 4 symbols on one diagram.
- The 4 symbols. Each end has 2 symbols. The one 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.
Common mistakes
- Reading the symbol at the wrong end. The symbol next to a box tells you how many of that box link to one row at the other end. The crow's foot sits by Employee because one department has many employees, not the other way round.
- Mixing up the inner and outer symbols. The symbol nearest the box is the maximum and the one further out is the minimum. A circle never sits next to the box, because zero can't be a maximum.
- Guessing the minimum. Choosing between one or many and zero or many is a business rule, not a drawing choice. Ask whether a row can exist without the other one, for example a new starter with no leave requests yet.
- Putting the key on the wrong side. The ID goes in the table on the many side, so Employee holds DepartmentID and LeaveRequest holds EmployeeID. A Department table can't hold a list of employee IDs in one column.
When a business analyst uses this
In a data requirements workshop, a business analyst reads each relationship out loud to the business, for example "each employee has zero or one parking permit", and asks if it's true. Each answer is written down as a business rule, and the line ends are fixed before the data model goes for sign-off.
Questions people ask
What does a crow's foot symbolise in an ER diagram?
The 3-pronged crow's foot means many. If it sits next to Employee on the Department line, one department can have many employees.
What is crow's foot database notation?
It's a way of drawing ER diagrams where the symbols on each end of a line show how many rows of one table can link to another. In Visio, Crow's Foot Database Notation is also the name of the stencil that has these shapes.
What are the 4 crow's foot notation symbols?
One and only one (2 bars), one or many (a bar and a crow's foot), zero or one (a circle and a bar) and zero or many (a circle and a crow's foot).
