How to generate NULL with equal values using the NULLIF() function?

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

SELECT NULLIF('yes', 'yes'), NULLIF('yes', 'no');
  -- +----------------------+---------------------+
  -- | NULLIF('yes', 'yes') | NULLIF('yes', 'no') |
  -- +----------------------+---------------------+
  -- | NULL                 | yes                 |
  -- +----------------------+---------------------+

Reference information of the NULLIF() function:

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

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

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⇑ MySQL Functions for Miscellaneous Purposes

⇑⇑ MySQL Function References