How to delete unpopular tags in WordPress via phpMyAdmin
If you are running an old blog undoubtedly you may have accumulated quite a good number of tags over the many years, thus your database has become cluttered with excess data.
This following SQL query will solve your problems.
DELETE FROM wp_terms WHERE term_id IN (SELECT term_id FROM wp_term_taxonomy WHERE count < 3 );
This will delete any tags that has been used less than 3 times, please note this query will only work in WordPress versions 4.0 or higher