SELECT * FROM wp_terms wt
INNER JOIN wp_term_taxonomy wtt ON wt.term_id=wtt.term_id
INNER JOIN wp_term_relationships wtr ON wtr.term_taxonomy_id=wtt.term_taxonomy_id
LEFT JOIN wp_posts wp ON wp.ID=wtr.object_id
WHERE
taxonomy='post_tag'
AND ID IS null
AND NOT EXISTS(SELECT * From wp_terms wt2
                INNER JOIN wp_term_taxonomy wtt2 ON wt2.term_id=wtt2.term_id WHERE wtt2.parent=wt.term_id)

ORDER BY name

If you run a query to delete old posts manually from MySQL in a WordPress database, the old tags will remain and appear in your tag cloud/listing. This allows you to identify those unused tags, and then delete them with an additional query.

WordPress Query to Identify Unused Tags

wordpress

MySQL