Syntax:
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<i>operand</i> IN (<i>subquery</i>|<i>value list</i>)
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An IN conditional expression returns true if the <i>operand</i> is found from result of the <i>subquery</i>
or if its in the specificied comma separated <i>value list</i>, hence the IN expression is always false if the result of the subquery
is empty.

When <i>value list</i> is being used there must be at least one element in that list.

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FROM C1 WHERE C1.col1 IN (FROM C2(col1));

FROM User WHERE User.id IN (1,3,4,5)
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The keyword IN is an alias for = ANY. Thus, these two statements are equal:
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FROM C1 WHERE C1.col1 = ANY (FROM C2(col1));
FROM C1 WHERE C1.col1 IN    (FROM C2(col1));
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