Calculate time with sometimes an 00:00:00 field

@kvdd identified the underlying cause: zero/placeholder date/time values in the table were breaking the arithmetic. Two practical, complementary paths fix this reliably — make the query tolerant of missing values, and fix the stored data/type so future rows cannot introduce the same problem.

A tolerant-query pattern converts any "zero" datetime into NULL (so it behaves like "no pause") and uses seconds arithmetic to add/subtract times safely. Example approach:

SELECT SEC_TO_TIME(
  COALESCE(TIME_TO_SEC(TIMEDIFF(hour_end, hour_begin)), 0)
  + COALESCE(TIME_TO_SEC(TIMEDIFF(hour_end_o, hour_begin_o)), 0)
  - COALESCE(TIME_TO_SEC(TIME(NULLIF(hour_pause,'0000-00-00 00:00:00'))), 0)
) AS total
FROM hour;

This treats zero-date placeholders as "no pause" and falls back to 0 seconds when fields are missing. See MySQL date/time functions for the used helpers (TIME_TO_SEC / SEC_TO_TIME / TIMEDIFF) and the null-handling function (COALESCE).

For a long-term fix, normalize the schema and data: convert meaningless zero-dates to NULL, use TIME for time-of-day or store durations as integer seconds, and prevent zero-dates from being inserted. Example maintenance steps:

UPDATE hour
SET hour_pause = NULL
WHERE hour_pause = '0000-00-00 00:00:00';

ALTER TABLE hour
  MODIFY hour_pause TIME NULL DEFAULT '00:00:00';

To avoid future zero-date inserts, enable appropriate SQL modes or validate at the application layer (SQL mode flags). Extra notes: if shifts can span midnight, add 24h when end < start or use DATETIME arithmetic; storing durations as seconds makes aggregation (SUM) and comparisons much simpler.