Crow's Foot Notation

A way of describing a entity-relationship diagram that is used in the logical database design and physical database design stage.

Entities §

All entities and relationships (with attributes) are represented as boxes

Attributes §

Added into an entity

Attribute Type	Icon
Mandatory (NOT NULL)	Blue Diamond
Optional	Empty Diamond
Derived	Surrounded with []
Multivalued	Surrounded with {}
Composite	Each subattribute is inside a ()
Key	Lightbulb

Derived Attributes only exist up to the Logical Database Design stage

They disappear in the physical database design stage, as they can be calculated from other attributes and do not need to be represented directly.

Relationships §

All relationships that do not contain attributes in Crow’s Foot Notation are represented directly using links.

Else, each relationship is represented as an entity, like in the relational data model

Relationship Type	Notation
Default	Associations have dashed lines
Identifiying	Associations have solid lines
Unary	Associations point back to the entity

Associations §

Crow's foot notation work's opposite to Chen's Notation

Unlike Chen’s Notation, crow’s foot applies it’s restrictions to the entity away from the symbols.

Association Type	Notation	Cardinality (On the opposite entity)
Default (Optional + Many)		$0\dots n$
Key Constraint (Optional One)		$0\dots 1$
Total Participation Constraint (Mandatory Many)		$1\dots n$
Total + Key Constrain (Mandatory One)		$1\dots 1$

Examples §

Weak Entity §

Remember that a weak entity is one that can only be uniquely reference using it’s parent entity:

• The weak entity has a key constraint and a total participation constraint
• The relationship is known as identifying

We can use crow’s foot notation to describe a weak entity as such:

Notice how the key constraint and participation constraint (the double lines) are away from the Weak Entity (closer to the Strong Entity).