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

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

SELECT help_category_id, STDDEV_POP(help_topic_id), COUNT(help_topic_id)
  FROM mysql.help_topic GROUP BY help_category_id;
  -- +------------------+---------------------------+----------------------+
  -- | help_category_id | STDDEV_POP(help_topic_id) | COUNT(help_topic_id) |
  -- +------------------+---------------------------+----------------------+
  -- |                1 |                       0.5 |                    2 |
  -- |                2 |        10.254954004414309 |                   35 |
  -- |                3 |         84.70569561304855 |                   59 |
  -- |                4 |                       0.5 |                    2 |
  -- |                5 |        1.6996731711975943 |                    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_POP() 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_POP(help_topic_id) OVER w
  FROM mysql.help_topic 
  WINDOW w AS (PARTITION BY help_category_id);
  -- +---------------+------------------+----------------------------------+
  -- | help_topic_id | help_category_id | STDDEV_POP(help_topic_id) OVER w |
  -- +---------------+------------------+----------------------------------+
  -- |             0 |                1 |                              0.5 |
  -- |             1 |                1 |                              0.5 |
  -- |             2 |                2 |               10.254954004414309 |
  -- |             6 |                2 |               10.254954004414309 |
  -- |             7 |                2 |               10.254954004414309 |
  -- |             8 |                2 |               10.254954004414309 |
  -- |             9 |                2 |               10.254954004414309 |
  -- ...
  -- +---------------+------------------+----------------------------------+

Reference information of the STDDEV_POP() function:

STDDEV_POP(expr): std
  Returns the population standard deviation of expr (the square root
  of VAR_POP()). If there are no matching rows, STDDEV_POP() returns NULL.

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

Related MySQL functions:

• STD() - Synonym for STDDEV_POP()
• STDDEV() - Synonym for STDDEV_POP()
• STDDEV_SAMP() - Sample Standard Deviation

⇒ STDDEV_SAMP() - Sample Standard Deviation

⇐ STDDEV() - Synonym for STDDEV_POP()

⇑ MySQL Functions on Aggregation Groups

⇑⇑ MySQL Function References