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Chapter 5. Relational Database Design > Relationship Optionality

Relationship Optionality

Relationship cardinality answers a fairly simple question: Given an entity A, how many instances, at most, of another entity B might potentially be linked to a given instance of A? The answer could be “zero” (in which case there's actually no relationship), but in general the answer is either “one” or “many” (in other words, more than one).

It can be useful to know one additional piece of information about a relationship. This is what's called the relationship's optionality. This information is not strictly necessary for a complete ERD, but it can be very useful information to gather.


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