Question

I was wondering how this query works:

SELECT empname FROM Employee WHERE not exists (
    SELECT projid FROM Project WHERE not exists (
        SELECT empid, projid FROM Assigned WHERE empid = Employee.empid and projid = Project.projid
    )
)

It is supposed to return names of all employees who are assigned to every project and it does work however I am getting confused as to how/why it works correctly.

Schema is:

Employee(empID INT,empName VARCHAR(100),job VARCHAR(100),deptID INT,salary INT);
Assigned(empID INT,projID INT,role VARCHAR(100));
Project(projID INT,title VARCHAR(100),budget INT,funds INT);

I am new to SQL so a detailed/simple explanation would be appreciated.

Answer 1

When I need to try to understand what’s going on, I look for the inner-most query and work my way outwards. In your case, let’s start with:

SELECT empid, projid 
FROM Assigned 
WHERE empid = Employee.empid and projid = Project.projid

This is matching all records in the Assigned table where the empid and projid are in the previous tables (hence the Employee.empid and Project.projid).

Assume there are 5 projects in the Projects table and Employee1 is assigned to each. That would return 5 records. Also assume Employee2 is assigned to 1 of those projects, thus returning 1 record.

Next look at:

SELECT projid FROM Project WHERE not exists (
        ...
    )

Now this says for those found records in the previous query (Employee1 with 5 projects and Employee2 with 1 project), select any projid from the Project table where there aren’t any matches (not exists) from the previous query. In other words, Employee1 would return no projects from this query but Employee2 would return 4 projects.

Finally, look at

 SELECT empname FROM Employee WHERE not exists (
        ...
    )

Just as with the 2nd query, for any records found in the previous query (no records to match those employees with all projects such as Employee1 and some records if the employee isn’t assigned to every project such as Employee2), select any employee from the Employee table where there aren’t any matches (again, not exists). In other words, Employee1 would return since no projects were returned from the previous query, and Employee2 would not return, since 1 or more projects were returned from the previous query.

Hope this helps. Here’s some additional information about EXISTS:

http://dev.mysql.com/doc/refman/5.0/en/exists-and-not-exists-subqueries.html

And from that article:

What kind of store is present in all cities?

SELECT DISTINCT store_type FROM stores s1   WHERE NOT EXISTS (
    SELECT * FROM cities WHERE NOT EXISTS (
      SELECT * FROM cities_stores
       WHERE cities_stores.city = cities.city AND cities_stores.store_type = stores.store_type));

The last example is a double-nested NOT EXISTS query. That is, it has
a NOT EXISTS clause within a NOT EXISTS clause. Formally, it answers
the question “does a city exist with a store that is not in Stores”?
But it is easier to say that a nested NOT EXISTS answers the question
“is x TRUE for all y?”

Good luck.