How to convert a time value into seconds using the TIME_TO_SEC() function?

TIME_TO_SEC(time) is a MySQL built-in function that converts a time value into seconds using the (s + 60*m + 60*60*h) formula. For example:

SELECT TIME_TO_SEC(NOW(3)), NOW();
  -- +---------------------+---------------------+
  -- | TIME_TO_SEC(NOW(3)) | NOW()               |
  -- +---------------------+---------------------+
  -- |               75921 | 2023-11-15 21:05:21 |
  -- +---------------------+---------------------+
1 row in set (0.00 sec)

SELECT TIME_TO_SEC('21:05:21'), 21+60*05+60*60*21;
  -- +-------------------------+-------------------+
  -- | TIME_TO_SEC('21:05:21') | 21+60*05+60*60*21 |
  -- +-------------------------+-------------------+
  -- |                   75921 |             75921 |
  -- +-------------------------+-------------------+

SELECT TIME_TO_SEC('100:00:00');
  -- +--------------------------+
  -- | TIME_TO_SEC('100:00:00') |
  -- +--------------------------+
  -- |                   360000 |
  -- +--------------------------+

Reference information of the TIME_TO_SEC() function:

TIME_TO_SEC(time): int
  converts a time value into seconds using the (s + 60*m + 60*60*h) formula.

Arguments, return value and availability:
  time: Required. The time value to be converted.
  int: Return value. The number of seconds from the given time.
  Available since MySQL 4.

⇒ TO_DAYS() - Converting Date to Days

⇐ TIME_FORMAT() - Formatting Time

⇑ MySQL Functions on Date and Time

⇑⇑ MySQL Function References