How much higher is the salary of Andrew Fuller than that of Nancy Davolio?
how much higher = SUBTRACT(SUM(Salary WHERE LastName = 'Fuller' and FirstName = 'Andrew'), SUM(Salary WHERE LastName = 'Davolio' and FirstName = 'Nancy'));
SELECT ( SELECT Salary FROM Employees WHERE LastName = 'Fuller' AND FirstName = 'Andrew' ) - ( SELECT Salary FROM Employees WHERE LastName = 'Davolio' AND FirstName = 'Nancy' ) AS RESULT