WHERE drops UNKNOWN and FALSE alike — the root cause behind half the P01/P04 traps.

WHERE keeps a row only when the predicate is TRUE; it drops FALSE and UNKNOWN alike. A comparison against NULL is UNKNOWN, so any plain <> / = filter silently discards the NULL rows — the root cause behind many filtering and aggregation surprises.

SELECT
  id
FROM
  tickets
WHERE
  priority <> 'high'
ORDER BY
  id;

SELECT
  id
FROM
  tickets
WHERE
  priority IS DISTINCT FROM 'high'
ORDER BY
  id;

The 'not high priority' queue loses every ticket with a blank priority — WHERE drops the UNKNOWN rows along with the FALSE ones.

When NULL rows should survive a not-equal filter, use IS DISTINCT FROM, or add OR col IS NULL explicitly.