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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 example calculates the sample standard deviation of five grade values: 
SELECT ROUND(STDDEV_SAMP(grade::double precision), 3) AS stddev
FROM UNNEST(ARRAY[4.0, 3.7, 3.3, 2.7, 2.7]) AS t(grade);
stddev double null
0.585

Rows: 1Execution time: 6.55ms