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I'm trying to figure out the difference between the single-bar and double-bar cardinality indicators as in the figure below:

Crow's foot example

Some sources seem to use one notation while others use the other. I can't tell if they actually refer to different kinds of relationships of if the difference is only in notation and they mean the same thing.

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    Possible duplicate of: stackoverflow.com/questions/33781451/… – Doug Deden Jul 2 at 18:51
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    I'm not seeing a consensus in the answers to the first question but u/dosenmatter suggests that "one" really means {0, 1} whereas "one-and-only-one" means {1}. This would make sense to me even though it results in the kind of strange one =/= {1}. It also seems to indicate that "one" is equivalent to "zero-or-one". I guess the single-bar cardinality indicator is ambiguous, although as u/Christophe's answer in the second question points out, this can sometimes be useful. – ahh_real_numbers Jul 3 at 17:00

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