从列值的硬编码列表（枢轴问题？）找到的第一行

Question

我试图从“第一”行满足某些标准，其中“第一”是由一个硬编码的列表中定义的提取列值列值。不幸的是，我是一名SQL业余爱好者。我正在使用DB2。

我能做到这一点低效这样的：

SELECT COALESCE(
    (SELECT COL FROM SOMETABLE WHERE X = "txt1" AND Y = "txt2" AND COL = 'A'), 
    (SELECT COL FROM SOMETABLE WHERE X = "txt1" AND Y = "txt2" AND COL = 'B'), 
    (SELECT COL FROM SOMETABLE WHERE X = "txt1" AND Y = "txt2" AND COL = 'C'), 
    (SELECT COL FROM SOMETABLE WHERE X = "txt1" AND Y = "txt2" AND COL = 'D') 
) 

这里的问题是，SOMETABLE是非常大的。所以不是我想这样做：

SELECT COALESCE(
    (SELECT COL FROM T WHERE COL = 'A'), 
    (SELECT COL FROM T WHERE COL = 'B'), 
    (SELECT COL FROM T WHERE COL = 'C'), 
    (SELECT COL FROM T WHERE COL = 'D') 
) 
FROM 
    (SELECT COL FROM SOMETABLE WHERE X = "txt1" AND Y = "txt2") AS T 

这是无效的，因为我不能引用的T台。

假设高于T包含下列行：

'E' 
'B' 
'D' 

然后我想挑“B”，因为它是在COALESCE语句中指定的第一个值。

我知道我可以做这样的事情得到一定程度接近我想要的东西：

SELECT 
    (CASE COL WHEN 'A' THEN COL ELSE NULL END), 
    (CASE COL WHEN 'B' THEN COL ELSE NULL END), 
    (CASE COL WHEN 'C' THEN COL ELSE NULL END), 
    (CASE COL WHEN 'D' THEN COL ELSE NULL END) 
FROM 
    (SELECT COL FROM SOMETABLE WHERE X = "txt1" AND Y = "txt2") AS T 

这样产生的结果是：

- - - - 
- - - D 
- B - - 

现在我想聚集或“扁平化”到这个单排，像这样：

- B - D 

然后，我会简单地凝聚在列。

任何建议，将不胜感激！

Answer 1

我想我找到了解决方案。

SELECT COALESCE(
    MAX(CASE COL WHEN 'A' THEN COL ELSE NULL END), 
    MAX(CASE COL WHEN 'B' THEN COL ELSE NULL END), 
    MAX(CASE COL WHEN 'C' THEN COL ELSE NULL END), 
    MAX(CASE COL WHEN 'D' THEN COL ELSE NULL END)) 
FROM 
    (SELECT COL FROM SOMETABLE WHERE X = "txt1" AND Y = "txt2") AS T 

请让我知道这是愚蠢的还是错误的。

Answer 2

如果我正确理解你的问题，你可以

SELECT COALESCE( 
    (SELECT COL FROM T WHERE COL = 'A'), 
    (SELECT COL FROM T WHERE COL = 'B'), 
    (SELECT COL FROM T WHERE COL = 'C'), 
    (SELECT COL FROM T WHERE COL = 'D')) 
FROM (SELECT COL FROM SOMETABLE WHERE X = "txt1" AND Y = "txt2") AS T 

转换为CTE为下：

WITH T AS (
    SELECT COL FROM SOMETABLE WHERE X = "txt1" AND Y = "txt2" 
) 
SELECT COALESCE( 
    (SELECT COL FROM T WHERE COL = 'A'), 
    (SELECT COL FROM T WHERE COL = 'B'), 
    (SELECT COL FROM T WHERE COL = 'C'), 
    (SELECT COL FROM T WHERE COL = 'D')) 
FROM T; 

或类似这样的东西。我无法测试它，因为我没有DB2数据库。

Answer 3

排序提供您的硬编码值，以便适用于您的问题，那么：

SELECT COL 
FROM SOMETABLE 
WHERE X = "txt1" 
AND Y = "txt2" 
ORDER BY col 
fetch first 1 rows only 

Answer 4

你是非常接近这个...

SELECT COALESCE(
    MAX(CASE COL WHEN 'A' THEN COL ELSE NULL END), 
    MAX(CASE COL WHEN 'B' THEN COL ELSE NULL END), 
    MAX(CASE COL WHEN 'C' THEN COL ELSE NULL END), 
    MAX(CASE COL WHEN 'D' THEN COL ELSE NULL END)) 
FROM 
    (SELECT COL FROM SOMETABLE WHERE X = "txt1" AND Y = "txt2") AS T 

你不需要聚结（可以” t不需要子查询（where子句总是先评估）

SELECT MAX(CASE WHEN COL = 'A' THEN COL ELSE NULL END) AS A, 
     MAX(CASE WHEN COL = 'B' THEN COL ELSE NULL END) AS B, 
     MAX(CASE WHEN COL = 'C' THEN COL ELSE NULL END) AS C, 
     MAX(CASE WHEN COL = 'D' THEN COL ELSE NULL END) AS D 
    FROM SOMETABLE 
WHERE X = "txt1" 
    AND Y = "txt2" 

某些数据库支持执行类似（但不完全相同）事情的ANSI SQL标准PIVOT功能。这听起来不像DB2/400有许多高级功能。

Joe