STDDEV()

Overview

The STDDEV() aggregate function calculates the sample standard deviation of a set of numeric values. Standard deviation measures the dispersion or spread of data points around the mean.

Syntax

The syntax for this function is as follows:

STDDEV(expression)

Parameters

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

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,
  length int,
  rating 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);

This query below uses the STDDEV() function to calculate the sample standard deviation for the length column:

SELECT
    STDDEV(length) AS LengthStdDev
FROM film;

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

   lengthstddev    
-------------------
 38.38092740197003
(1 row)