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sql - 使用 case 语句时的 Group By 子句问题

问题描述

我试图总结每个不同发件人全年的数量，并将年份作为字段。我正在使用 case 语句来执行此操作。 

select distinct sender,
  case
    when extract(month from (month)) = 1
      then sum(volume)
  end as January,
  case
    when  extract(month from (month)) = 2
      then sum(volume)
  end as February,
  case
    when extract(month from (month)) = 3
      then sum(volume)
  end as March
from [lifetime_data]
group by 1

当我运行这个时，我收到一个错误，要求我按月分组，月份不是我选择的列之一，我只在我的情况下使用它......我该如何解决这个问题？

标签： sqlpostgresql

解决方案

我认为你想要条件聚合——case表达式是一个参数sum()：

select sender,
       sum(case when extract(month from (month)) = 1 then volume end) as January,
       sum(case when extract(month from (month)) = 2 then volume end) as February,
       sum(case when extract(month from (month)) = 3 then volume end) as March
from [lifetime_data]
group by 1

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