What are WordPress orphaned user meta and how to clean them?

The user meta data is the information you provide to viewers about each user. This information usually includes additional data that is not stored in the users table of WordPress. In some cases, some user meta data information become orphaned and do not belong to any user. They are then called “orphaned usermeta” and should be cleaned since they are not useful.
To clean these orphaned usermeta, you can either use our plugin Advanced Database Cleaner, or execute the following sql code (which is used by the plugin) directly via your phpMyAdmin:

DELETE FROM usermeta WHERE user_id NOT IN (SELECT ID FROM users)