UNION 查询问题

Question

我的查询是：

SELECT BRAND,BRAND_GROUP, SUB_BRAND ,SUM(INCOME) AS TOTAL_INCOME FROM
"tema".MMT WHERE BRAND_GROUP IS NULL AND SUB_BRAND IS NULL GROUP BY
BRAND,BRAND_GROUP,SUB_BRAND

UNION
SELECT BRAND,BRAND_GROUP, SUB_BRAND ,SUM(INCOME) AS TOTAL_INCOME FROM
"tema".BGT WHERE BRAND_GROUP IS NULL AND SUB_BRAND IS NULL GROUP BY 
BRAND,BRAND_GROUP,SUB_BRAND;

我的输出是：

BRAND BRAND_GROUP SUB_BRAND TOTAL_INCOME 
----- ----------- --------- ------------ 
GBS   NULL        NULL             10000
SWG   NULL        NULL             10000
GBS   NULL        NULL             20000
STG   NULL        NULL             20000
GTS   NULL        NULL             30000

问题是我有 2 类品牌，而我只想拥有 1 类。像这样：

Brand       Brand_Group     Sub_brand       Total_Income
GBS                  -          -           30000
STG                  -          -           20000
GTS                  -          -           30000
SWG                  -          -           10000

有人可以帮我一个ideea吗？

Answer 1

合并后的总和（希望 DB2 这样做）：

SELECT  BRAND, BRAND_GROUP, SUB_BRAND, SUM(INCOME) AS TOTAL_INCOME
FROM    (SELECT BRAND, BRAND_GROUP, SUB_BRAND, INCOME
         FROM   "tema".MMT
         WHERE  BRAND_GROUP IS NULL
         AND    SUB_BRAND IS NULL
         UNION ALL
         SELECT BRAND, BRAND_GROUP, SUB_BRAND, INCOME
         FROM   "tema".BGT
         WHERE  BRAND_GROUP IS NULL
         AND    SUB_BRAND IS NULL) AS T
GROUP BY BRAND, BRAND_GROUP, SUB_BRAND;

就一个问题。为什么需要BRAND_GROUPand SUB_BRAND（因为它们都是NULL）？