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“Name of each Employee who works on all the Project controlled by department number 5”

Which of the following is correct?

A)Select Fname,Lname from Employee where not exists((Select Pnumber from Project where Dnum=5) EXCEPT (Select Pno from Works_On where Ssn=ESsn));

B)Select LName,Fname from Employee where Ssn in(Select ESsn from Works_On Bwhere B.Pno=All(Select Pnumber from Project where Dnum=5));

Which option is correct and why?

in Databases by (817 points) | 15 views

@Satbir @Shaik help plz

A is correct.
(Select Pnumber from Project where Dnum=5) - this returns all projects of Dept. No 5
Select Pno from Works_On where Ssn=ESsn - this returns all the projects that a person works on, for each employee.

When we take the difference of the two and this returns as empty, this means that either the person works on all projects of Dept 5 or a superset of it.
Not exists is true when the condition is empty, and hence, this tuple is selected.

When will this fail? When there are projects of dept 5 which the employee doesn't work on. In that case, not exists will not be empty and the query will not be selected.

When we take the difference of the two and this returns as empty,

we are not taking difference. right?? How empty relation output  " who works on all the Project controlled by department number 5"??

I havenot got.

Give some example 

EXCEPT in SQL is used to take difference.
give some example plz.

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