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Computes the sample standard deviation of all non-NULL numeric values produced by an expression. The sample standard deviation measures how spread out values are in a sample by calculating the square root of the average of squared deviations from the sample mean, using a correction for sample size. For information about the population standard deviation, which estimates the spread of values in the full population, see STDDEV_POP. Alias: STDDEV

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Syntax

{ STDDEV | STDDEV_SAMP }(<expression>) [FILTER ([WHERE] <condition>)]

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Parameters

ParameterDescriptionSupported input types
<expression>An expression producing numeric values for which to calculate the sample standard deviation.REAL, DOUBLE PRECISION
<condition>An optional boolean expression to filter rows used in aggregationBOOL

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Return Type

STDDEV_SAMP returns a result of type DOUBLE PRECISION.

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Special cases

  • If there is at most one non-NULL input value, the result is NULL.
  • If the input contains an Inf or NaN value, the result will be NaN.

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Example

The following code creates an exams table with a grade column of type DOUBLE PRECISION, and inserts five grade values into it: 
CREATE TABLE exams (grade DOUBLE PRECISION);
INSERT INTO exams VALUES (4.0), (3.7), (3.3), (2.7), (2.7);
The following code calculates the sample standard deviation of the grade values from the exams table, rounds the result to three decimal places, and returns it as stddev: 
SELECT ROUND(STDDEV_SAMP(grade), 3) as stddev from exams;
Returns The previous code returns the following result:
stddev (DOUBLE PRECISION)
0.585