当表格仅包含一行时，StDev()函数返回Null [英] StDev() function returns Null when table contains only one row

问题描述

我正在尝试使用StDev函数并获得空白结果.我将其用作...

I am trying to use the StDev function and am getting blank results. I am using it as...

SELECT StDev(fldMean) FROM myTable

其中fldMean包含2.3的值，并且应求值为0，但我只是得到一个空结果.我似乎无法理解该函数中如何使用表达式，微软的手册确实没有帮助.

Where fldMean contains a value of 2.3 and should evaluate to 0 but instead I am simply getting an empty result. I can't seem to understand how expressions are to be used in the function, Microsoft's manual really didn't help.

推荐答案

SELECT StDev(fldMean) FROM myTable

如果[myTable]只有一行，则

将返回Null，因为无法从单个观察值计算出标准偏差.在收到有意义的结果之前，该表中至少需要两行.如果您想将Null值强制设置为零，则可以使用

will return Null if [myTable] has only one row because the Standard Deviation cannot be computed from a single observation. You will need at least two rows in that table before you can receive a meaningful result. If you want to force the Null value to zero you can use

SELECT Nz(StDev(fldMean), 0) FROM myTable

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