如何在Group By中使用Case Statement列

Question

正如问题所述,我正在尝试在列结果中制定一个包含case语句的查询,然后我想在查询的group by语句中包含该列.举一个具体的例子,这里几乎没有我的查询:

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

来到这里之前,我查了几个网站,包括这个网站,试着解决我的问题.我已尝试CASE在链接网页中显示关键字之后添加另一个别名,但没有运气.我继续收到的错误消息如下:

[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 

有没有其他人遇到我面临的问题,并能够GROUP BY在CASE声明的结果上使用a ？任何帮助,将不胜感激.哦,DB2版本是z/OS实例,版本10(DSN10015)

Answer 1

别名不能在GROUP BY中使用,因为当GROUP BY发生时,别名尚未定义:

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

你可以解决这个问题:

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

或者只使用您在GROUP BY中的SELECT中使用的案例

Answer 2

您可以按原样使用案例,如下所示:

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

或者像这样使用子查询:

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