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I'm trying to wrap my head around the identifying/non-identifying relationships in Crow's Foot notation. I have read in most places that an identifying relationship means that the FK is a part of the entity's PK. However, wouldn't that be the same as simply having the FK be non-null and not include it in the primary key?

For example: enter image description here

To me this is an identifying relationship, since a Student cannot exist without a University. Though, I do not have plans on including the FK from Student to University in the Student's PK.

  • My head wraps differently than your head. To me, a student is a person, and would still be a person, even with no university. And could have an email address. I guess the question is whether the student's id includes info about which university the student studies at. Also, what if a student studies at more than one university? All this depends on your analysis of your case. – Walter Mitty Feb 25 '17 at 12:27
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According to this post, as pointed out by mustaccio, while the formal way of showing an identifying relationship is by including the FK in the PK in the child table, this is not always done since it might not be practical to have big PKs that include the foreign key(s).

The logical meaning is simply that the child cannot exist without the parent, and as long as it is true, the relationship is identifying.

Therefore, in the example I stated, if we assume that all the students in our database have to be enrolled in a university, the relationship between a Student and a University is classified as identifying, since a Student cannot exist without a University.

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