What is unsafe query?

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zhu mi answered
I met about this question's answer like this :This query is syntactically correct. However, it asks for all tuples S such that S is not in (the given instance of) Sailors. The set of such S tuples is obviously infinite, in the context of infinite domains such as the set of all integers. This simple example illustrates an unsafe query. It is desirable to restrict relational calculus to disallow unsafe queries.I'm sured if it works for you.

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