มีสินค้ากี่ตัวที่มีลักษณะที่เรียกว่า “ฮอต”?
How many products have the characteristic named "hot"?
df_products = pd.DataFrame(columns=['product_id'])
df_characteristics = pd.DataFrame(columns=['characteristic_id', 'characteristic_name'])
df_product_characteristics = pd.DataFrame(columns=['product_id', 'characteristic_id'])
df_products[df_products['product_id'].isin(df_product_characteristics[df_product_characteristics['characteristic_id'].isin(df_characteristics[df_characteristics['characteristic_name'] == 'hot']['characteristic_id'])]['product_id'])].shape[0]