How to evaluate a field expression on n-row before the current row in the current result set window using the LAG() function?

LAG(exp, n, default) is a MySQL built-in window function that evaluates a field expression on n-row before the current row in the current result set window. For example:

SELECT help_topic_id AS tip, help_category_id AS cid,
  name, LAG(name, 2, NULL) OVER w
  FROM mysql.help_topic
  WINDOW w AS (PARTITION BY help_category_id);
  -- +-----+-----+----------------+----------------------------+
  -- | tip | cid | name           | LAG(name, 2, NULL) OVER w  |
  -- +-----+-----+----------------+----------------------------+
  -- |   0 |   1 | HELP_DATE      | NULL                       |
  -- |   1 |   1 | HELP_VERSION   | NULL                       |
  -- |   2 |   2 | AUTO_INCREMENT | NULL                       |
  -- |   6 |   2 | BIT            | NULL                       |
  -- |   7 |   2 | TINYINT        | AUTO_INCREMENT             |
  -- |   8 |   2 | BOOLEAN        | BIT                        |
  -- |   9 |   2 | SMALLINT       | TINYINT                    |
  -- |  10 |   2 | MEDIUMINT      | BOOLEAN                    |
  -- ...
  -- +-----+-----+----------------+----------------------------+

Reference information of the LAG() function:

LAG(exp, n, default): val
  Evaluates a field expression on n-row before the current row
  in the current result set window.

Arguments, return value and availability:
  exp: Required. The field expression to be evaluated.
  n: Optional. Default is 1. The number of rows lagging the current row.
  default: Optional. Default is NULL. The default value for non-existing rows.
  val: Return value. The exp value on n-row before the current row.
  Available since MySQL 8.

Related MySQL functions:

• LAG() - N-Row before Current Row within Window
• LEAD() - N-Row after Current Row within Window

⇒ LAST_VALUE() - Last Value of Result Set Window

⇐ FIRST_VALUE() - First Value of Result Set Window

⇑ MySQL Functions on Result Set Windows

⇑⇑ MySQL Function References