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I'm a bit surprised there's no built-in function to say "this value is X-sigma away from the mean"
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No one can possibly foresee all of the different calculations one might want to do with values.
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I am still wondering if there's a nice function or something that returns the sigma value for any given set of records?
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Not sure what you mean. Sigma is just standard deviation so you would use that function.
See STDDEV in the SQL Language doc
http://docs.oracle.com/cd/E11882_01/server.112/e26088/functions178.htm#sthref1566
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STDDEV returns the sample standard deviation of expr, a set of numbers. You can use it as both an aggregate and analytic function. It differs from STDDEV_SAMP in that STDDEV returns zero when it has only 1 row of input data, whereas STDDEV_SAMP returns null.
Oracle Database calculates the standard deviation as the square root of the variance defined for the VARIANCE aggregate function.
This function takes as an argument any numeric data type or any nonnumeric data type that can be implicitly converted to a numeric data type. The function returns the same data type as the numeric data type of the argument.