Sum up the number sales ids handled by employees called Morningstar, Heather and Dean.
SUM = ADD(SUM(SalesID WHERE FirstName = 'Morningstar'), SUM(SalesID WHERE FirstName = 'Heather'), SUM(SalesID WHERE FirstName = 'Dean'));
SELECT SUM(IIF(T2.FirstName = 'Morningstar', 1, 0)) + SUM(IIF(T2.FirstName = 'Heather', 1, 0)) + SUM(IIF(T2.FirstName = 'Dean', 1, 0)) AS num FROM Sales AS T1 INNER JOIN Employees AS T2 ON T1.SalesPersonID = T2.EmployeeID