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

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

SELECT help_category_id, VAR_SAMP(help_topic_id), COUNT(help_topic_id)
  FROM mysql.help_topic GROUP BY help_category_id;
  -- +------------------+-------------------------+----------------------+
  -- | help_category_id | VAR_SAMP(help_topic_id) | COUNT(help_topic_id) |
  -- +------------------+-------------------------+----------------------+
  -- |                1 |                     0.5 |                    2 |
  -- |                2 |      108.25714285714287 |                   35 |
  -- |                3 |       7298.762711864405 |                   59 |
  -- |                4 |                     0.5 |                    2 |
  -- |                5 |        4.33333333333333 |                    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 |
  -- +------------------+---------------+

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

SELECT help_topic_id, help_category_id, 
  VAR_SAMP(help_topic_id) OVER w
  FROM mysql.help_topic 
  WINDOW w AS (PARTITION BY help_category_id);
  -- +---------------+------------------+--------------------------------+
  -- | help_topic_id | help_category_id | VAR_SAMP(help_topic_id) OVER w |
  -- +---------------+------------------+--------------------------------+
  -- |             0 |                1 |                            0.5 |
  -- |             1 |                1 |                            0.5 |
  -- |             2 |                2 |             108.25714285714287 |
  -- |             6 |                2 |             108.25714285714287 |
  -- |             7 |                2 |             108.25714285714287 |
  -- |             8 |                2 |             108.25714285714287 |
  -- |             9 |                2 |             108.25714285714287 |
  -- ...
  -- +---------------+------------------+--------------------------------+

Reference information of the VAR_SAMP() function:

VAR_SAMP(expr): var
  Returns the sample variance of expr. That is, the denominator is the
  number of rows minus one. If there are no matching rows, VAR_SAMP()
  returns NULL.

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

Related MySQL functions:

• VAR_POP() - Population Standard Variance

⇒ VARIANCE() - Synonym for VAR_POP()

⇐ VAR_POP() - Population Standard Variance

⇑ MySQL Functions on Aggregation Groups

⇑⇑ MySQL Function References