| [`COUNT(DISTINCT)`](https://dev.mysql.com/doc/refman/5.7/en/aggregate-functions.html#function_count-distinct) | 返回不同值的数目 | | [`SUM()`](https://dev ...
-- 4. Display the total price before discount and after discount. (quantity * unit_price) AS total_price_before_discount, ((quantity * unit_price) - COALESCE(discount ...