你可以使用EXISTS:

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

或INNER JOIN:

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

或 IN 运算符:

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