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Overview

The STDDEV_SAMP() aggregate function calculates the sample standard deviation of a set of numeric values. This function measures how much the values deviate from their mean, assuming the data is a sample of a larger population.

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Syntax

The syntax for this function is as follows:

STDDEV_SAMP(expression)

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Parameters

  • expression: numeric expression or column for which the sample standard deviation is calculated

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Example

For the needs of this section, we’re going to use a simplified version of the film table from the Pagila database, containing only the title, length and rating columns. The complete schema for the film table can be found on the Pagila database website.

DROP TABLE IF EXISTS film;
CREATE TABLE film (
  title text NOT NULL,
  rating int,
  length int
);
INSERT INTO film(title, length, rating) VALUES
  ('ATTRACTION NEWTON', 83, 5),
  ('CHRISTMAS MOONSHINE', 150, 7),
  ('DANGEROUS UPTOWN', 121, 4),
  ('KILL BROTHERHOOD', 54, 3),
  ('HALLOWEEN NUTS', 47, 5),
  ('HOURS RAGE', 122, 7),
  ('PIANIST OUTFIELD', 136, 7),
  ('PICKUP DRIVING', 77, 3),
  ('INDEPENDENCE HOTEL', 157, 7),
  ('PRIVATE DROP', 106, 4),
  ('SAINTS BRIDE', 125, 3),
  ('FOREVER CANDIDATE', 131, 7),
  ('MILLION ACE', 142, 5),
  ('SLEEPY JAPANESE', 137, 4),
  ('WRATH MILE', 176, 7),
  ('YOUTH KICK', 179, 7),
  ('CLOCKWORK PARADISE', 143, 5);

The query below uses the STDDEV_SAMP() function to calculate the sample standard deviation for the length column where rating is greater than or equal to 4:

SELECT
    STDDEV_SAMP(length) AS LengthSampleStdDev
FROM film
WHERE rating >= 4;

By running the code above we will get the following output:

 lengthsamplestddev 
--------------------
  34.92503746251735
(1 row)
STDDEV_POPVARIANCE