如何查询UNION查询的结果

Question

如果问题很愚蠢，谢谢您的时间和歉意。我有一个联合查询的结果，如下所示。如您所见，A和C都具有value2而不是NULL。我如何只能选择B？我会提供我的sql尝试，但我用尽了所有想法。我尝试了count（）和sum（）等方法，但是我总是选择A B C，而我只想获得B。 谢谢。

value1 value2 
  A    NULL 
  A    1
  B    NULL
  C    2
  C    3
  C    NULL

非常感谢您的帮助。我得到了答案以及更多！你们都很棒！

Answer 1

这是您想要的吗？

select value1
from t
group by value1
having count(value2) = 0;

Answer 2

我不确定您是卡在UNION还是GROUP BY上，所以这是我的示例：

SELECT VALUE1 FROM 
(
    --put your UNION query in here, I'm just building your example data
    SELECT 'A' AS VALUE1, NULL AS VALUE2
    UNION
    SELECT 'A' AS VALUE1, 1 AS VALUE2
    UNION
    SELECT 'B' AS VALUE1, NULL AS VALUE2
    UNION
    SELECT 'C' AS VALUE1, 2 AS VALUE2
    UNION
    SELECT 'C' AS VALUE1, 3 AS VALUE2
    UNION
    SELECT 'C' AS VALUE1, NULL AS VALUE2
) AS THETABLE
GROUP BY VALUE1
HAVING COUNT(VALUE2) = 0

结果：

有很多方法可以做到这一点，这只是其中一种。

Answer 3

您是否熟悉子查询或CTE？它们使您可以将结果集像表一样对待。

所以...

WITH CTE
  ( SELECT * -- Put your union query hear
      FROM YourUnionQuery
  )
-- Then this is the actual transformation you're looking for
    SELECT C1.*  
      FROM CTE C1
     WHERE NOT EXISTS
             ( SELECT 1
                 FROM CTE C2
                WHERE C2.value2 IS NOT NULL
                  AND C2.value1 = C2.value1
             )