crowfoots

Crowfoots, also known as crow’s foot notation, are symbols used in entity-relationship diagrams to express the cardinality and participation of relationships between data entities. They are a common tool in relational database design and data modeling, valued for providing a compact visual summary of how entities relate to one another.

In crowfoot notation, each end of a relationship line carries a symbol that conveys how many instances of one entity may be related to instances of the other. A straight bar generally indicates exactly one, a crow’s foot indicates many, and a circle indicates zero (optionality). By combining these marks at both ends, designers can express common cardinalities such as one-to-one, one-to-many, and many-to-many, including variations like zero-or-one or zero-to-many.

History and context: Crowfoot notation emerged in the 1970s as an accessible alternative to other ER diagram styles. It gained popularity among database professionals for its intuitive visual language and its direct mapping to relational schemas, where “many” sides often become foreign-key relationships. It is often contrasted with Chen’s notation and other ER approaches such as Bachman or IDEF1X.

Usage and interpretation: In practice, crowfoots are used to plan and document database schemas. A one-to-many relationship, for example, is typically represented with a bar at the one end and a crow’s foot at the many end. The many side usually corresponds to the table containing the foreign key. Variants exist across tools and teams, and some diagrams may include additional notations for mandatory versus optional participation or for identifying relationships.

See also: entity-relationship model, crow’s foot notation, IDEF1X.