Question

我是SQL的新手，所以请考虑这个 noob 问题。此外，我很尴尬地承认我无法在Google中搜索正确的关键字，而且我已经没时间了，所以我决定在这里问一下。

代码：

select
 *,
 price * quantity as [Total price],
 case
   when [Total price]>100 and [Total price]<= 200 then '2%'
   when [Total price]>200 and [Total price]<= 300 then '3%'
   when [Total price]>300 and [Total price]<= 400 then '4%'
   else '0%'
 end as tax
from
grocery

正如你所看到的，我想要做的是尝试在执行SQL语句时创建一个临时变量但是，这个给了我错误

Error 1: could not prepare statement [1 no such column: Total price]

我怎么能这样做？

Answer 1

快速的方法是使用CTE（公用表表达式）。这允许您预先计算一些值，然后引用查询正文中的值。

如果使用CTE的陈述不是批次中的第一件事，则需要使用;结束前面的内容：

 ;
 With Totals as
 (
    select  *,
            price * quantity as [Total price],
    from    grocery
 )
 select  *
    ,  case
         when [Total price]>100 and [Total price]<= 200 then '2%'
         when [Total price]>200 and [Total price]<= 300 then '3%'
         when [Total price]>300 and [Total price]<= 400 then '4%'
        else '0%'
     end as tax
 from
   Totals

顺便提一下，问题的根源在于您无法定义表达式，然后在查询中使用表达式 by name 。您可以重复使用表达式，但不能使用名称：

select  x + 1 as Expr1
   ,    (x + 1) * 2  as Expr2
from   Table1

这样可行，但以下不会工作：

select x + 1 as Expr1
   ,   Expr1 * 2 as Expr2   -- This won't work
from   Table1

Answer 2

SELECT *,
 case
   when [Total price]>100 and [Total price]<= 200 then '2%'
   when [Total price]>200 and [Total price]<= 300 then '3%'
   when [Total price]>300 and [Total price]<= 400 then '4%'
   else '0%'
 end as tax
FROM (SELECT *, price * quantity as [Total price]
      FROM grocery) A

OR

select
 *,
 price * quantity as [Total price],
 case
   when price * quantity >100 and price * quantity<= 200 then '2%'
   when price * quantity>200 and price * quantity<= 300 then '3%'
   when price * quantity>300 and price * quantity<= 400 then '4%'
   else '0%'
 end as tax
from
grocery

SQL：创建临时变量

2 个答案: