SQL Join condition clause - sql

Question

I have database with as diagram shows (see picture below)
My task is to show total value of orders handled by each Employees.
I have SQL statement:
SELECT e.FirstName,
e.LastName,
SUM(od.Quantity * od.UnitPrice * (1-od.Discount))
FROM Orders AS o
JOIN Employees AS e
ON o.EmployeeID = e.EmployeeID
JOIN [Order Details] AS od
ON o.OrderID = od.OrderID
GROUP BY e.FirstName,e.LastName
I have problem with further steps. I need to limited results only for those employees which:
A) Have employees under them
B) Don't have any employees under them
I know It concern field ReportsTO in Employees table, but I don't know how get proper SQL clause. I am supposed to do with "EXISTS" or self-join ?
Thank You.

Answer 1

Yes, Just use Exists.
Where Exists(Select 1 from Employees where ReportsTo = e.EmployeeId)

Answer 2

Use a self join to try to find someone reporting to him
SELECT e.FirstName,
e.LastName,
SUM(od.Quantity * od.UnitPrice * (1-od.Discount))
FROM Orders AS o
INNER JOIN Employees AS e
ON o.EmployeeID = e.EmployeeID
LEFT JOIN Employees as under -- self join
ON e.EmployeeID = under.ReportTo
INNER JOIN [Order Details] AS od
ON o.OrderID = od.OrderID
GROUP BY e.FirstName,e.LastName
HAVING MAX(under.ReportTo) IS NULL -- If doesnt find a match mean no one subordinate
-- MAX(under.ReportTo) IS NOT NULL -- mean have at least one subordinate

Answer 3

With a self join you can potentially create duplicates which would be difficult to debug due to the GROUP BY. EXISTS with a correlated sub query in the WHERE clause would be how I would accomplish this.
SELECT *
FROM Employees e
WHERE EXISTS(
SELECT 1
FROM Employees _e
WHERE _e.ReportsTo = e.EmployeeID)
In your query:
SELECT e.FirstName,
e.LastName,
SUM(od.Quantity * od.UnitPrice * (1-od.Discount))
FROM Orders AS o
JOIN Employees AS e
ON o.EmployeeID = e.EmployeeID
JOIN [Order Details] AS od
ON o.OrderID = od.OrderID
WHERE /*NOT*/ EXISTS(
SELECT 1
FROM Employees _e
WHERE _e.ReportsTo = e.EmployeeID)
GROUP BY e.FirstName,e.LastName