Made easy test series

Question

Emp (Eid, name, age)

Project(Pid, Pname, budget)

Works(Eid, Pid)

Select Eid from Emp E where age>30 not exists (Select Pid from Project P where Pname="database" and not exists (select Pid from Works W where W.Eid=E.Eid and W.Pid=E.Pid));

Above query retrieves employees whose

a) age more than 30 and works for every project  with project name database.

b) age more than 30 and works for some project with project name database.

c) age more than 30 and not works for every project with project name database.

d) age more than 30 and not works for any project with project name database. 

here Eid and Pid in every table is primary key.

answer given is option a, but i have doubt because am getting option b. someone please clarify my doubt with example.

Answer 1

This is an example of Double Nested NOT EXISTS query. That is, it has a NOT EXISTS clause within a 

NOT EXISTS clause. Formally, it answers the question “does an employee exist with projectname Database and a project that is not in Works”? But it is easier to say that a nested NOT EXISTS answers the question 

“is x TRUE for all y?”

In our case, that is equivalent to "Find the employees whos age is more than 30 and works for every project  with project name database"

Comments

Is it true for every double not exists query?? What if there were 3 not exists in a query then??

---

I have seen max. 2 Not exists Clause together within Same Query.

If you have any examples with 3 , kindly bring them here. I will Look forward to it.