# Source: https://docs.oxla.com/sql-reference/sql-functions/aggregate-functions/stddev.md

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# 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:

```sql  theme={null}
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
<a href="https://www.postgresql.org/ftp/projects/pgFoundry/dbsamples/pagila/pagila/" target="_blank">Pagila</a> database website.

```sql  theme={null}
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:

```sql  theme={null}
SELECT
    STDDEV(length) AS LengthStdDev
FROM film;
```

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

```sql  theme={null}
   lengthstddev    
-------------------
 38.38092740197003
(1 row)
```