Zkuste toto:
SELECT T1.products_bought, COUNT(T2.cnt) AS total
FROM (
SELECT '<5' AS products_bought, 0 AS a, 4 AS b
UNION ALL
SELECT '5-10', 5, 10
UNION ALL
SELECT '>10', 11, 999999
) T1
LEFT JOIN
(
SELECT COUNT(DISTINCT ProductID) AS cnt
FROM ORDERS
GROUP BY CustomerID
) T2
ON T2.cnt BETWEEN T1.a AND T1.b
GROUP BY a, b
Výsledek:
products_bought total
<5 3
5-10 0
>10 0