MySQL Vælg fra kategori i tabel y hvor Count (af kategoriens produkt fra en anden tabel) er større end nul

Du kan bruge EXISTS :

SELECT category, categoryID 
FROM categories AS c
WHERE EXISTS (SELECT 1 
              FROM products AS p
              WHERE p.categoryID  = c.categoryID )

eller en INNER JOIN :

SELECT DISTINCT category, categoryID 
FROM categories AS c
INNER JOIN products AS p ON p.categoryID  = c.categoryID

eller IN operatør:

SELECT category, categoryID 
FROM categories 
WHERE categoryID IN (SELECT categoryID FROM products)