One and only one

How to read it

Crow’s foot cardinality marks come in pairs. The mark nearer the entity box gives the maximum, and the mark farther out gives the minimum. A bar means one in either position, so two bars mean at least one and at most one: exactly one. Database texts call this side of the relationship mandatory. In a textbook example, a line with two bars at entity A and a Zero or many mark at entity B reads: each A has zero or more Bs, and each B belongs to exactly one A.

The mark describes the entity it touches, read from the other end of the line. It is easily confused with Zero or one, where an open circle takes the place of the outer bar.

Also searched as: crow’s foot one to one, mandatory one, one and only one symbol.

Look-alikes

The Zero or one mark has a circle and a bar: the relationship is optional. The One or many mark keeps the outer bar but ends in a three-pronged crow’s foot, so more than one record may be linked.

Differences by notation

The same constraint is written in other ways. In (min, max) notation it is (1,1) beside the entity, and in UML-style multiplicity it is 1..1. In Chen notation as Elmasri and Navathe draw it, a “1” beside the line means zero or one; exactly one takes a 1 together with a double line for total participation. In Richard Barker’s notation, used in Oracle’s CASE tools, there are no bars: a solid half-line with no crow’s foot is read as “must be … one and only one”.