我正在编写一个SQL查询,其中返回的一些列需要根据相当多的条件进行计算。
我目前使用的是嵌套的case语句,但它变得越来越混乱。有没有更好的(更有条理和/或更具可读性的)方法?
(我使用的是Microsoft SQL Server,2005)
一个简化的例子:
SELECT
col1,
col2,
col3,
CASE
WHEN condition
THEN
CASE
WHEN condition1
THEN
CASE
WHEN condition2
THEN calculation1
ELSE calculation2
END
ELSE
CASE
WHEN condition2
THEN calculation3
ELSE calculation4
END
END
ELSE
CASE
WHEN condition1
THEN
CASE
WHEN condition2
THEN calculation5
ELSE calculation6
END
ELSE
CASE
WHEN condition2
THEN calculation7
ELSE calculation8
END
END
END AS 'calculatedcol1',
col4,
col5 -- etc
FROM table
发布于 2012-07-13 01:17:23
把所有的箱子都包起来。
SELECT
col1,
col2,
col3,
CASE
WHEN condition1 THEN calculation1
WHEN condition2 THEN calculation2
WHEN condition3 THEN calculation3
WHEN condition4 THEN calculation4
WHEN condition5 THEN calculation5
ELSE NULL
END AS 'calculatedcol1',
col4,
col5 -- etc
FROM table
发布于 2013-05-31 19:45:44
您可以组合多个条件来避免这种情况:
CASE WHEN condition1 = true AND condition2 = true THEN calculation1
WHEN condition1 = true AND condition2 = false
ELSE 'what so ever' END,
发布于 2009-02-03 01:58:00
我个人就是这样做的,限制了嵌入的CASE表达式。我还会添加注释来解释正在发生的事情。如果它太复杂了,就把它分解成函数。
SELECT
col1,
col2,
col3,
CASE WHEN condition THEN
CASE WHEN condition1 THEN
CASE WHEN condition2 THEN calculation1
ELSE calculation2 END
ELSE
CASE WHEN condition2 THEN calculation3
ELSE calculation4 END
END
ELSE CASE WHEN condition1 THEN
CASE WHEN condition2 THEN calculation5
ELSE calculation6 END
ELSE CASE WHEN condition2 THEN calculation7
ELSE calculation8 END
END AS 'calculatedcol1',
col4,
col5 -- etc
FROM table
https://stackoverflow.com/questions/505747
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