Finding Missing Matches

One common use of `LEFT JOIN` is to find rows that do not have a match.

Start with the table you want to keep, `LEFT JOIN` the related table, then filter to rows where the right side is `NULL`.

For example, to find departments without employees:

SELECT d.name AS department_name
FROM departments AS d
LEFT JOIN employees AS e
  ON d.id = e.department_id
WHERE e.id IS NULL

This pattern is very useful for spotting missing data and gaps in relationships.