Zero or one
Zero or one: optional, and at most one. On a crow’s foot ER diagram, a circle and a short bar at the end of a relationship line mean each record at the other end has no matching record, or one, in the entity beside them.
Easily confused with
How to read it
The bar, nearer the entity box, sets the maximum at one. The open circle, farther out, sets the minimum at zero, so the link is optional. A textbook example is an employee who may manage a factory: not every employee manages one, and none manages two.
The circle is read as a zero, and is the detail that separates this mark from One and only one, which has a second bar in its place.
Also searched as: optional one, zero or one symbol.
Look-alikes
The One and only one mark has two bars and no circle. The Zero or many mark keeps the circle but ends in a crow’s foot, for any number of records.
Differences by notation
In (min, max) notation the same constraint is (0,1), and in UML-style multiplicity 0..1. In Chen notation as Elmasri and Navathe draw it, a plain “1” already means zero or one. Barker’s notation, used by Oracle’s CASE tools, has no circle: a dashed half-line means the relationship is optional (“may be”), and no crow’s foot means one and only one.
Read next
- Next in ER diagram symbolsOne or manyOne or more. On a crow’s foot ER diagram, a bar and a three-pronged crow’s foot at the end of a relationship line mean each record at the other end has at least one, and possibly many, matching records in the entity beside them.
- The same shape in African flagsFlag of EthiopiaEthiopia’s flag is green, yellow and red in equal horizontal stripes with a blue disc in the centre holding a yellow star of equal lines with yellow rays. Ethiopia’s Flag Proclamation makes the green labour, fertility and development, the yellow hope, justice and equality, and the red sacrifice and heroism for freedom and equality; the blue disc is peace and the star the unity of Ethiopia’s nations, nationalities and peoples.
More ER diagram symbols
The same shape elsewhere
Browse symbols drawn with circles, lines or squares and rectangles.
Sources
- A Practical Introduction to Databases: 2.4 ERD alternatives and variations, Christopher Painter-Wakefield, 2022, Runestone Academy, 2.4.1 Crow’s foot notation; 2.4.1 Crow’s foot notation; 2.4.2.1 Cardinality ratios and participation
- Entity Relationship Modeling in Database Management Systems, page 4, Richard Holowczak, 2013, updated 2022, Holowczak.com Tutorials, Visible Analyst notation
- Richard Barker (data modeling notations), David C. Hay, 1999, Essential Strategies, Inc., Cardinality/optionality
- A Practical Introduction to Databases: 2.2 Entity-relationship diagrams, Christopher Painter-Wakefield, 2022, Runestone Academy, 2.2.1.5 Cardinality ratios and participation
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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