I'd recommend that instead of joining the same table twice you use SUM
together with Iif
, like this:
SELECT
Customers.[Customer ID],
Customers.[First Name],
Customers.[Last Name],
SUM(Iif(r.[Valid Referral] = True, 1, 0)) AS c,
Sum(Iif(r.[Used Our Service] = True, 1, 0)) AS c2
FROM Customers
INNER JOIN Referrals r ON Customers.[Customer ID] = r.[Referred From]
WHERE (r.[Valid Referral])=True
GROUP BY Customers.[Customer ID], Customers.[First Name], Customers.[Last Name]