sql - 如何在单个查询中从一个表中的计算值进行除法

Question

我有以下 SQL 查询，我想计算一个派生字段 A/B：

select center, month, sum(countA) as A, sum(countB) as B, A/B 
from counter
group by center, month

但是，这句话抛出了一个错误，我希望不再重复总和。有没有办法在单个查询上做到这一点？

Answer 1

不幸的是，别名在同一个 select 子句中是未知的。您可以按以下方式进行计算：

select center, month, sum(countA) as A, sum(countB) as B, sum(countA) / sum(countB)
from counter
group by center, month;

您也可以在子查询中执行此操作：

select center, month, A, B, A/B
from (select center, month, usm(countA) as A, sum(countB) as B
      from counter
      group by center, month
     ) t

Answer 2

您可以执行以下操作：

select center, month, A, B, A/B from
(
    select center, month, sum(countA) as A, sum(countB) as B
    from counter
    group by center, month
)

但是如果 B 为零，您仍然可能会收到错误消息。为了解决这个问题，试试这个：

select center, month, A, B, case when B = 0 then 0 else A/B end from
(
    select center, month, sum(countA) as A, sum(countB) as B 
    from counter
    group by center, month
)