mysql - 尝试在 mySQL 的新列中将计算四舍五入到小数点后两位

Question

所以我正在努力...

Write a SELECT statement that returns these column names and data from the Products table:

• product_name
• list_price
• discount_percent
• discount_amount
  • A column that’s calculated from the previous two columns
• discount_price
  • A column that’s calculated from the previous three columns

Round the discount_amount and discount_price columns to 2 decimal places.

Sort the result set by discount price in descending sequence.

Use the LIMIT clause so the result set contains only the first 5 rows.

到目前为止我已经

USE my_guitar_shop;

SELECT product_name, list_price, discount_percent,
             CONCAT(discount_percent / 100 * list_price)
                AS discount_amount
FROM products

我能够返回 discount_amount，但无法将该列四舍五入到小数点后两位。

我该如何返回第二列？比如

ROUND(list_price - discount_amount, 2) 
AS discount_price  

它说它无法识别discount_amount？

Answer 1

使用ROUND()而不是CONCAT。

ROUND(discount_percent / 100 * list_price, 2)

第一个参数是要舍入的数字。第二个参数是四舍五入到多少位小数。

http://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html#function_round

Blockquote How would I also go about returning a second column? Such as

ROUND(list_price - discount_amount, 2) 
AS discount_price  

您不能重复使用这样的别名。我只是重复这个表达

ROUND(list_price - (discount_percent / 100 * list_price), 2) 
AS discount_price