如何在組中使用Case Statement列

Question

正如問題所述，我試圖制定一個在列結果中有一個case語句的查詢，然後我想通過查詢的組中包含該列聲明。舉一個具體的例子，這裏是所有稍微分析一下我的查詢看起來像：

SELECT SOME_TABLE_ALIAS.COLUMN1, OTHER_TABLE_ALIAS.COLUMN2, 
CASE 
    WHEN SOME_TABLE_ALIAS.COLUMN3 IS NOT NULL THEN 'A' 
    ELSE 'B' 
END AS CASE_COLUMN 
FROM SOME_TABLE SOME_TABLE_ALIAS 
... (other table joins and where clauses) 
GROUP BY SOME_TABLE_ALIAS.COLUMN1, OTHER_TABLE_ALIAS.COLUMN2, CASE_COLUMN 

前來到這裏，我檢查了幾個網站，包括this one，試圖解決我的問題。我試過在關聯網頁中顯示CASE關鍵字之後添加另一個別名，但沒有運氣。該錯誤消息我繼續收到如下：

[Error] Script lines: 127-151 ---------------------- 
CASE_COLUMN IS NOT VALID IN THE CONTEXT WHERE IT IS USED. SQLCODE=-206, SQLSTATE=42703, DRIVER=3.53.71 

有沒有其他人遇到我面對並已經能夠使用上CASE語句的結果GROUP BY的問題是什麼？任何幫助，將不勝感激。哦，還有DB2版本的z/OS實例，第10版（DSN10015）

Answer 1

別名是不可用的組使用BY，因爲當GROUP BY發生別名沒有定義尚未：

Here's the order: 
1.FROM 
2.WHERE 
3.GROUP BY 
4.HAVING 
5.SELECT 
6.ORDER BY 

您可以解決與：

SELECT column1,column2,case_column 
FROM (
SELECT SOME_TABLE_ALIAS.COLUMN1, OTHER_TABLE_ALIAS.COLUMN2, 
CASE 
    WHEN SOME_TABLE_ALIAS.COLUMN3 IS NOT NULL THEN 'A' 
    ELSE 'B' 
END AS CASE_COLUMN 
FROM SOME_TABLE SOME_TABLE_ALIAS 
... (other table joins and where clauses) 
) a 
GROUP BY COLUMN1, COLUMN2, CASE_COLUMN 

或者只是使用您在SELECT在GROUP使用由

Answer 2

您可以通過使用情況下，該組中，這樣的：

SELECT SOME_TABLE_ALIAS.COLUMN1, OTHER_TABLE_ALIAS.COLUMN2, 
CASE 
    WHEN SOME_TABLE_ALIAS.COLUMN3 IS NOT NULL THEN 'A' 
    ELSE 'B' 
END AS CASE_COLUMN 
FROM SOME_TABLE SOME_TABLE_ALIAS 
... (other table joins and where clauses) 
GROUP BY SOME_TABLE_ALIAS.COLUMN1, OTHER_TABLE_ALIAS.COLUMN2, 
CASE 
    WHEN SOME_TABLE_ALIAS.COLUMN3 IS NOT NULL THEN 'A' 
    ELSE 'B' 
END 

或使用子查詢像這樣：

select COLUMN1, COLUMN2, CASE_COLUMN 
from (
    SELECT SOME_TABLE_ALIAS.COLUMN1, OTHER_TABLE_ALIAS.COLUMN2, 
    CASE 
     WHEN SOME_TABLE_ALIAS.COLUMN3 IS NOT NULL THEN 'A' 
     ELSE 'B' 
    END AS CASE_COLUMN 
    FROM SOME_TABLE SOME_TABLE_ALIAS 
    ... (other table joins and where clauses) 
) a 
GROUP BY COLUMN1, COLUMN2, CASE_COLUMN