How to calculate the density rank of the sorting field expression in the current result set window using the DENSE_RANK() function?

DENSE_RANK() is a MySQL built-in window function that calculates the density rank of the sorting field expression in the current result set window. For example:

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

Reference information of the DENSE_RANK() function:

DENSE_RANK(): val
  Returns the density rank of the sorting field expression
  in the current result set window.

Arguments, return value and availability:
  val: Return value. The density rank of the sorting field.
  Available since MySQL 8.

Related MySQL functions:

• CUME_DIST() - Cumulative Distribution of Sorted Values
• DENSE_RANK() - Density Rank of Sorted Values
• PERCENT_RANK() - Rank Percentage of Sorted Values
• RANK() - Vale Rank of Sorted Values

⇒ FIRST_VALUE() - First Value of Result Set Window

⇐ CUME_DIST() - Cumulative Distribution of Sorted Values

⇑ MySQL Functions on Result Set Windows

⇑⇑ MySQL Function References