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R875 / Generalization

Select Action is a Single Select or Many Select

From a purely relational perspective, there is no reason to distinguish based on flow cardinality. The instance references in a flow are a set of tuples. This could be the empty set or a set with any number of instance reference tuples. Operations on instance references generally operate on all tuples in the set.

From a modeler's perspective, however, certain rules are observed and expressed when expressing actions. For example, selection using an identifier must return zero or one instance. The modeler knows this and the resultant actions are easier to understand in this context. A modeler might express something along the lines of "Get me the Aircraft corresponding to this tail number". The modeler could instead say "Get me the set of Aircraft instances corresponding to this tail number" which is technically correct, but not as meaningful in the real world.