Crow's Foot Notation
In ERD, there is a wide range of notations used by data architects for denoting the relationship and cardinality between the data entities
One of three relationships can exist between two entities.
One to One
One to Many
Many to Many
Example
- A car needs a driver (One to One)
- A car has four wheels (One to Many)
- A car can carry more than one person (One to Many)
- A driver is allowed to drive more than one car and a car can be driven by more than one driver (Many to Many).
A cardinality is the number of rows a table can have in the relationship.
The possible cardinalities are
One and Only One
One or Many
Zero or One or Many
Zero or One
A car can carry more than one person, the car will carry minimum one person, which is the driver. Also it can carry many passengers.
The car has a One to (One or Many) relationship.
Summary
A car can have one and only gearknop – One to (One and Only One)
A driver can drive more than one vehicle but at the same time he doesn’t need to own a car and can use public transport. In this instance the driver drives zero vehicles – One to (Zero or One or Many).
A car can be declared off the road and doesn’t require a tax disc. In this instance, a car doesn't have a tax disc - One to (Zero or One).
A car owner or the owner’s spouse or any comprehensive licence holder can drive the owner’s car – (Many to One).
Crow Foot Notation Symbols
Data model
Conceptual data model - draft ER diagram
Logical data model - Uafhængig af tech. Detaljegrad lidt større
Physical data model - Final, klar til at implementere
One teacher can have many Courses One to many Optional A course needs one teacher, and only one Many to one