将两列相乘并在新列中显示 [英] Multiply two columns and display in new one

问题描述

我有3个名为 customers ， cars ， carrent 的表.我想要的就是将 rentorder.days 与 cardetail.rentday 乘以并在 rentorder.totalrent 中显示该值.我无法实现这一目标.我怎样才能做到这一点.请提出任何建议.

I am having 3 tables named customers,cars,carrent. All i want is to multiply rentorder.days with cardetail.rentday and show the value in rentorder.totalrent. I am unable to achieve this. How can i do this. Any suggestions please.

SQL

SELECT customers.*, 
       cardetail.carname, 
       cardetail.model, 
       cardetail.company, 
       cardetail.color, 
       cardetail.rentday, 
       rentorder.days, 
       rentorder.totalrent 
FROM   rentorder 
       INNER JOIN customers 
               ON customers.custid = rentorder.custid 
       INNER JOIN cardetail 
               ON cardetail.id = rentorder.carid 

推荐答案

只需将 rentday 与 days 相乘即可得到计算值 totalrent 选择列表.

Just multiply rentday with days to get the computed value totalrent in Select list.

尝试一下

SELECT customers.*, -- Not sure this is allowed in [Mysql]
       cardetail.carname, 
       cardetail.model, 
       cardetail.company, 
       cardetail.color, 
       cardetail.rentday, 
       rentorder.days, 
       cardetail.rentday * rentorder.days AS totalrent 
FROM   rentorder 
       INNER JOIN customers 
               ON customers.custid = rentorder.custid 
       INNER JOIN cardetail 
               ON cardetail.id = rentorder.carid 

要保存数据，您需要使用内部联接语法

To save the data you need to use update from Inner Join syntax

update rentorder 
       INNER JOIN customers 
               ON customers.custid = rentorder.custid 
       INNER JOIN cardetail 
               ON cardetail.id = rentorder.carid 
SET
    rentorder.totalrent = cardetail.rentday * rentorder.days

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