As, i have shown that employee b
Will not be selected as it is not working on all the projects that are controlled by dept. 5 but in first query ,
IN is there which compares a
value ssn with a set (or multiset) of values essn and evaluates to TRUE if ssn is one of the elements in essn as when inner most query project return {10,25,15} then due to IN the essn 2 of employee name b will also be considered as it is working on project 10 and 15 as result of which the ssn will be match with essn 2 and the row with employee name b get printed