如何將聚合結果轉換爲列？

Question

請告訴我，如果你知道的術語來描述下列行動：

輸入

dfips dcounty  context sumton 
19001 Adair County mail 6521.79995560646 
19001 Adair County Rail 38411.5996840298 

輸出：

dfips dcounty  mail_sumton  rail_sumton 
19001 Adair County 6521.79995560646 38411.5996840298 

我想輸入轉換爲輸出，但我不知道如何描述這樣的行爲。我能想到的最好方法是將聚合結果轉換爲列。

Answer 1

一個pivot()的簡單交叉版本是這樣的：

select 
    dfips 
    , dcounty 
    , mail_sumton = sum(case when context = 'mail' then sumton else null end) 
    , rail_sumton = sum(case when context = 'rail' then sumton else null end) 
from t 
group by dfips, dcounty 

Answer 2

條件聚集

select 
    dfips, 
    dcounty, 
    sum(case when context = 'mail' then isnull(sumton,0) else null end) as mail_sumton, 
    sum(case when context = 'rail' then isnull(sumton,0) else null end) as rail_sumton, 
from yourTable 
group by 
    dfips, dcounty 

Answer 3

可以使用聚合函數sum（或max按你的需求）來實現這一目標。

select 
    dfips, 
    dcounty, 
    sum(case when context = 'mail' then sumton end) mail_sumton, 
    sum(case when context = 'Rail' then sumton end) rail_sumton 
from your_table 
group by 
    dfips, 
    dcounty 

Answer 4

或者，您可以使用查詢中的PIVOT功能 - 見文檔在https://technet.microsoft.com/en-us/library/ms177410(v=sql.105).aspx