SQL:在同一张表上组合四个 "where"子句

Question

我目前正在努力让它发挥作用。

SELECT        Y, SUM(N) AS sumDK, NULL AS sumFI, NULL AS sumNO, NULL AS sumSE
FROM            stat
WHERE        (C = 'Denmark')
GROUP BY Y
UNION ALL
SELECT        Y, NULL AS sumDK, SUM(N) AS sumFI, NULL AS sumNO, NULL AS sumSE
FROM            stat
WHERE        (C = 'Finland')
GROUP BY Y
UNION ALL
SELECT        Y, NULL AS sumDK, NULL AS sumFI, SUM(N) AS sumNO, NULL AS sumSE
FROM            stat
WHERE        (C = 'Norway')
GROUP BY Y
UNION ALL
SELECT        Y, NULL AS sumDK, NULL AS sumFI, NULL AS sumNO, SUM(N) AS sumSE
FROM            stat
WHERE        (C = 'Sweden')
GROUP BY Y

这个查询给了我一个像这样的矩阵:

Y | sumDK | sumFI | sumNO | sumSE
----------------------------------
2011| 25 | NULL | NULL | NULL 
2012| 5 | NULL | NULL | NULL 
2011| NULL | 18 | NULL |NULL 
2012| NULL | 5 | NULL |NULL 
......

如何执行只返回字段 NOT NULL 的查询？
提前致谢!

Answer 1

SELECT      Y, C, SUM(N) 'Sum'
FROM        stat
WHERE       C IN ( 'Denmark', 'Finland', 'Norway', 'Sweden' )
GROUP BY    Y, C