How to calculate the sample standard deviation of a field expression in result set groups using the STDDEV_SAMP() function?

STDDEV_SAMP(expr) is a MySQL built-in aggregate function that calculates the sample standard deviation of a field expression in result set groups. For example:

SELECT help_category_id, STDDEV_SAMP(help_topic_id), COUNT(help_topic_id)
  FROM mysql.help_topic GROUP BY help_category_id;
  -- +------------------+----------------------------+----------------------+
  -- | help_category_id | STDDEV_SAMP(help_topic_id) | COUNT(help_topic_id) |
  -- +------------------+----------------------------+----------------------+
  -- |                1 |         0.7071067811865476 |                    2 |
  -- |                2 |         10.404669281488138 |                   35 |
  -- |                3 |          85.43279646520068 |                   59 |
  -- |                4 |         0.7071067811865476 |                    2 |
  -- |                5 |          2.081665999466132 |                    3 |
  -- ...
  -- +------------------+----------------------------+----------------------+

SELECT help_category_id, help_topic_id
  FROM mysql.help_topic WHERE help_category_id = 5;
  -- +------------------+---------------+
  -- | help_category_id | help_topic_id |
  -- +------------------+---------------+
  -- |                5 |            40 |
  -- |                5 |            43 |
  -- |                5 |            44 |
  -- +------------------+---------------+

STDDEV_SAMP() is also a window function, you can call it with the OVER clause to calculate the population standard deviation of the given expression in the current window. For example:

SELECT help_topic_id, help_category_id, 
  STDDEV_SAMP(help_topic_id) OVER w
  FROM mysql.help_topic 
  WINDOW w AS (PARTITION BY help_category_id);
  -- +---------------+------------------+-----------------------------------+
  -- | help_topic_id | help_category_id | STDDEV_SAMP(help_topic_id) OVER w |
  -- +---------------+------------------+-----------------------------------+
  -- |             0 |                1 |                0.7071067811865476 |
  -- |             1 |                1 |                0.7071067811865476 |
  -- |             2 |                2 |                10.404669281488138 |
  -- |             6 |                2 |                10.404669281488138 |
  -- |             7 |                2 |                10.404669281488138 |
  -- |             8 |                2 |                10.404669281488138 |
  -- |             9 |                2 |                10.404669281488138 |
  -- ...
  -- +---------------+------------------+-----------------------------------+

Reference information of the STDDEV_SAMP() function:

STDDEV_SAMP(expr): std
  Returns the sample standard deviation of expr (the square root of
  VAR_SAMP().

Arguments, return value and availability:
  expr: Required. The field expression in result set groups.
  std: Return value. The sample standard deviation of the input expression.
  Available since MySQL 5.7.

Related MySQL functions:

• STDDEV_POP() - Population Standard Deviation

⇒ SUM() - Total Value in Group

⇐ STDDEV_POP() - Population Standard Deviation

⇑ MySQL Functions on Aggregation Groups

⇑⇑ MySQL Function References