Sådan bruges en temp-kolonne i where-klausulen

Brug HAVING i stedet:

Select
    product_brand,
    (CASE WHEN COUNT(product_brand)>50 THEN 1 ELSE 0 END) AS brand_count
  FROM products
  GROUP BY product_brand
  HAVING brand_count = 1

WHERE evalueres før GROUP BY . HAVING evalueres efter.