Answer by Gordon Linoff for Selecting ALL from a specific range

I think group_concat() is the simplest solution. Assuming no duplicates, you can get all the projects for department 5 as:

select group_concat(p.pnumber order by p.pnumber)from projects pwhere dnumber = 5;

Then, you can do the same thing for the employees and match them:

select e.ssn, group_concat(wo.pnumber order by wo.pnumber)from works_on wowhere wo.pnumber in (select p.pnumber from projects p where p.dnumber = 5)group by e.ssn;

And finally match the two, such as in a having clause:

select e.ssn, group_concat(wo.pnumber order by wo.pnumber) as projectsfrom works_on wowhere wo.pnumber in (select p.pnumber from projects p where p.dnumber = 5)group by e.ssnhaving projects = (select group_concat(p.pnumber order by p.pnumber)                   from projects p                   where dnumber = 5                  );

Another approach using relational logic would go as:

select e.ssnfrom employees e cross join     projects p left join     works_on wo     on e.ssn = wo.ssn and p.pnumber = wo.pnumberwhere p.dnumber = 5group by e.ssnhaving count(wo.snn) = count(*);   -- no NULL values