How to substitute a NULL value with a replacement value using the IFNULL() function?

IFNULL(val1, val2) is a MySQL built-in function that returns the second argument if the first argument is NULL, the first argument otherwise. For example:

SELECT IFNULL(NULL, 'replacement'), IFNULL('original', 'replacement');
  -- +-----------------------------+-----------------------------------+
  -- | IFNULL(NULL, 'replacement') | IFNULL('original', 'replacement') |
  -- +-----------------------------+-----------------------------------+
  -- | replacement                 | original                          |
  -- +-----------------------------+-----------------------------------+

Reference information of the IFNULL() function:

IFNULL(val1, val2): val
  Returns the second argument if the first argument is NULL, 
  the first argument otherwise.

Arguments, return value and availability:
  val1, val2: Required. The values to be selected from.
  val: Return value. The selected value.
  Available since MySQL 4.0.

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⇑ MySQL Functions for Miscellaneous Purposes

⇑⇑ MySQL Function References