Mysql 将两列相乘并以新列显示

我有三张桌子，分别是“顾客”、“汽车”和“卡伦特”。我只想将rentorder.days与cardetail.rentday相乘，并在rentorder.totalrent中显示值。我无法做到这一点。我怎样才能做到这一点。 有什么建议吗

SQL

只需将rentday与days相乘，即可获得选择列表中的计算值totalrent

试试这个

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，需要使用“从内部联接更新”语法

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在rentorder.totalrent中显示值-您只是想显示它，还是将其保存在rentorder表中？@PaulSpiegel我想保存它的Select customers.*，-不确定[Mysql]中允许这样做，-是的@VR如果我想保存值怎么办？选择customers.*，-不确定[Mysql]中是否允许这样做，-是的！-只有第一个。SELECT col1，table.*不起作用。@endo64我刚刚测试了SELECT productIdata，categories.*-它在MariaDB上起作用。我也经常在MySQL上这样做来调试查询。@endo64，您使用的是*而不是表别名，如t1.*-这是不同的。使用SELECT col1，*-*将包括col1-这意味着您将选择col1两次。

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 

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