如果爲空或空，請選擇不同的列

Question

我想選擇state或workstate列。首選使用state值。如果它爲空或空，那麼我想使用workstate值。

我有以下SQL：

SELECT   
COUNT(id) AS participants, 

COALESCE(NULLIF(state,''), workState, (select state from dbo.training where id =(SELECT top(1) trainingId 
    FROM dbo.Xref_Participant_Training 
    WHERE (participantId = dbo.Participant.id)))) as state 

FROM dbo.Participant 
WHERE EXISTS 
    (SELECT 1 
     FROM dbo.Xref_Participant_Training 
     WHERE (participantId = dbo.Participant.id) AND (dbo.Participant.country = 'United States')) 
GROUP by COALESCE(NULLIF(state,''), workState, (select state from dbo.training where id =(SELECT top(1) trainingId 
     FROM dbo.Xref_Participant_Training 
     WHERE (participantId = dbo.Participant.id)))) 

我收到以下錯誤：

Msg 144, Level 15, State 1, Line 15 
Cannot use an aggregate or a subquery in an expression used for the group by list of a GROUP BY clause. 

Answer 1

變化GROUP BY包括整個表達式

SELECT   
     COUNT(id) AS participants, 
     COALESCE(NULLIF(state,''), workState) as state 
FROM dbo.Participant 
WHERE EXISTS 
    (SELECT participantId 
     FROM dbo.Xref_Participant_Training 
     WHERE (participantId = dbo.Participant.id) AND (dbo.Participant.country = 'United States')) 
GROUP BY COALESCE(NULLIF(state,''), workState) 

爲您更新的查詢，當查詢變得複雜時，使用派生表或cte

爲派生表

select col1, col2, count(*) 
from 
(
    select col1, col2 = coalesce(col2, col3), col4, col5 
    from sometable 
) d 
group by col1, col2 

Answer 2

這個例子中的語法是將原始問題的答案。

的問題是，你需要把在group by整個表達式：

SELECT COUNT(id) AS participants, 
     COALESCE(NULLIF(p.state,''), p.workState) as state 
FROM dbo.Participant p 
WHERE EXISTS (SELECT 1 
       FROM dbo.Xref_Participant_Training pt 
       WHERE pt.participantId = p.id 
      ) AND 
     p.country = 'United States' 
GROUP BY COALESCE(NULLIF(p.state, ''), p.workState); 

與您的查詢的問題是，GROUP BY state真正意義GROUP BY p.state。但是，您想按照SELECT中定義的列進行聚合。

另外：

• 表的別名使查詢更容易編寫和閱讀。
• 您不需要EXISTS子查詢的列名稱。