Question

我想将同一表中两个子查询的结果除以不同 Postgres中的where条件

select (a / b) as result from 
    (select sum(price) as sales from transactions where order_type = 'return') as a,
    (select sum(price) as sales from transactions where order_type = 'sale') as b

Answer 1

我认为您不需要在其中使用别名，您可以选择部门本身，例如，尝试以下操作：

select (select sum(price) from transactions where order_type = 'return') / (select sum(price) from transactions where order_type = 'sale') as result;

在假设的transactions表中，如下所示：

 price | order_type
-------+------------
     4 | return
     5 | sale
     6 | return
(3 rows)

该查询将输出以下内容：

 result
--------
      2
(1 row)

因为（4 + 6）/ 5 = 2

HIH

Answer 2

您的代码应该没问题。您可以更通俗地写为：

select ( sum(price) filter (where order_type = 'return') /
         sum(price) filter (where order_type = 'sale')
       ) as result
from transactions;

这只会扫描transactions表一次。

如果没有销售，这样做的确有除零错误的风险。如果可能的话：

select ( sum(price) filter (where order_type = 'return') /
         nullif(sum(price) filter (where order_type = 'sale'), 0)
       ) as result
from transactions;

将结果与两个条件的子查询相除

2 个答案: