I am learning about the Crow's Foot notation, and there is something that I don't understand, some tutorials shows an Identifying Relationship as having a straight line, and a Non-Identifying Relationship as having a dashed line:

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While other tutorials shows both relationships as having a straight line:

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Are both notations correct?

  • 1
    What is this "other tutorials" you speak of? Can you provide one or more links? Nov 4, 2016 at 20:07
  • @Robert Harvey This tutorial uses straight lines for Non-Identifying Relationships: college.yukondude.com/2003_09_comp210/html/…, while this tutorial uses dashed lines for Non-Identifying Relationships: codeproject.com/Articles/878359/…
    – Tom
    Nov 4, 2016 at 20:46
  • That Yukondude article refers not to strong and weak relationships, but to strong and weak entities. I assume that he's indicating the Transcript as a weak entity with those little diagonal lines in the corners. But I also assume that's not necessarily standard notation. Nov 4, 2016 at 22:30
  • @Robert Harvey What is this notation called (where a Non-Identifying Relationship is represented using a dashed line and an Identifying Relationship is represented using a straight line)? Is it called IE Crow's Foot or Martin Crow's Foot or Martin / IE / Crow's Foot? And does it have an official documentation?
    – Tom
    Nov 5, 2016 at 13:18

1 Answer 1


The +O on the left side indicates it is non-identifying (optional) on the right side. This would be implemented as a nullable column on the right hand side. This is common to all tools I have worked with, and certainly the two tools you are working with.

The ends of the relationships have show two values (max and min) indicating the cardinality of the relationship for the table at the other end of the line. The min value has two possible cardinality values zero (O) or 1 (| or + ), The max value also has two possible diagrammed values 1 (| or +) or many (a crowsfoot). If either min value is zero, then the relationship is optional.

An identifying value must have a cardinality of (1-1). Some tools may show only one cardinality indicator for a (1-1) cardinality.

Some tools use dashed lines for optional relationships. Both notations are correct. I prefer the dashed lines format , especially when the diagram is large or busy. I have worked with tools that provide extensive capability to modify how the relationship lines are rendered such as modifying the linewidth, pattern and color.

  • 1
    I don't think so. Nov 4, 2016 at 22:28
  • @RobertHarvey See my edit for an explanation.
    – BillThor
    Nov 6, 2016 at 12:58
  • Are you saying that it has to be non-identifying on both sides (aka a zero on both sides) for the overall relationship to be called non-identifying (and therefore be represented by a dashed line)? Also, so by saying it is non-identifying, they are saying that a student does not have to belong to a school? Seems odd to me. How can you be a student if you are not enrolled in a school? Feb 24, 2017 at 18:59
  • @fgblomqvist By non-identifying I mean that School would not be used to identify the student. Student would never identify a school. A non-identifying relationship does not need to be optional on either end, and may be a mandatory relationship. As diagrammed, a school is not required to have students (new school with no assigned students). Nor does a student need to be assigned to a school (new student not yet assigned to a school).
    – BillThor
    Feb 25, 2017 at 5:45

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