有没有办法用 case 函数显示负值？

Question

问题：

提供一份报告，显示客户的全名（名字和姓氏一起）、山寨名称、山寨的地区名称、预订日期（格式为 2017 年 12 月 15 日）、预订价格、以及该小屋的预订价格与所有小屋的平均预订价格之间的差额（以 $yy.y 格式，例如 $25.6 格式，截断到最接近的小数点后 1 位），按全名降序排列。

我的回答：

SELECT CONCAT(Customer_FName," ",Customer_LName) AS 'Name', Cottage_Name, Region_Name, 
DATE_FORMAT(Booking_Date,"%D %M %Y" ) AS 'Date Booked', Booking_Price,
CASE 
    WHEN AVG(Booking_Price) < Booking_Price THEN Booking_Price - AVG(Booking_Price)
    ELSE -1 * (AVG(Booking_Price) - Booking_Price)
END AS 'Diff From Average'

FROM Customer

JOIN Booking USING(Customer_ID)
JOIN Cottage
JOIN Region
GROUP BY Name;

我的输出：

所需的输出必须显示负值而不是 0

我的问题：有谁知道可以与CASE... WHEN 执行相同操作并显示负值的函数？

SQLFiddle（包含数据和所有必要的文件）：http://sqlfiddle.com/#!9/8a40db/2/0

ERD：

问题提示：

Answer 1

只有零，因为平均值与预订价格相同

但是根本不需要你的理由

您最好使用正确的连接和 ON clqause

SELECT 
    CONCAT(Customer_FName," ",Customer_LName) AS 'Name'
    , Cottage_Name
     , Region_Name, 
     DATE_FORMAT(Booking_Date,"%D %M %Y" ) AS 'Date Booked'
    , Booking_Price
    ,
    CONCAT('$',ROUND((Booking_Price - AVG(Booking_Price)),1)) AS 'Diff From Average'
FROM Customer
    JOIN Booking USING(Customer_ID)
    JOIN Cottage
   JOIN Region
GROUP BY Name;



Name            Cottage_Name                        Region_Name     Date Booked         Booking_Price   Diff From Average
Bob Perkins     Atlantic Beach Resort Cottage 1     Halifax         10th January 2018   144             $0.0
Charlie Oscar   Atlantic Beach Resort Cottage 1     Halifax         15th January 2018   1176            $294.0
Eve Evenfield   Atlantic Beach Resort Cottage 1     Halifax         22nd December 2017  654             $0.0
Frank Mitchell  Atlantic Beach Resort Cottage 1     Halifax         15th January 2018   384             $0.0
Grace Damon     Atlantic Beach Resort Cottage 1     Halifax         1st January 2018    170             $0.0
Student DalFCS  Atlantic Beach Resort Cottage 1     Halifax         15th January 2018   584             $154.5

Answer 2

您的查询未正确聚合g，尽管 MySQL 对它的编译不够宽松。

如果您运行的是 MySQL 8.0，您可以使用窗口函数来实现：

select
    concat(c.Customer_FName, ' ', c.Customer_LName) AS Name, 
    b.Cottage_Name, 
    r.Region_Name, 
    DATE_FORMAT(Booking_Date,"%D %M %Y" ) AS Date_Booked, 
    b.Booking_Price,
    AVG(b.Booking_Price) OVER() - Booking_Price diff_from_average
from customer c
inner join booking b using(customer_id)
inner join region r using(??)

在早期版本中，您可以使用聚合查询来计算总体平均值：

select
    concat(c.Customer_FName, ' ', c.Customer_LName) AS Name, 
    b.Cottage_Name, 
    r.Region_Name, 
    date_format(Booking_Date,"%D %M %Y" ) AS Date_Booked, 
    b.Booking_Price,
    a.avg_booking_Price - Booking_Price diff_from_average
from customer c
inner join booking b using(customer_id)
inner join region r using(??)
cross join (select avg(booking_price) avg_booking_price from booking) a

旁注：

• 我添加了表别名并尝试在列前加上相关表 - 您可能需要查看

• 您在region 上缺少连接条件