Today I was working on tagging books in my Library, and wanted to filter by books that don't have a tag. I first tried mytag:"", but that didn't work. But, I was able to get it to work with the following filter:
* ANDNOT mytag:*
This will show you all your resources (*), as long is there is no text in the mytag field (AND NOT mytag:*). This can be helpful if you like tagging your resources and just want to see the resources you haven't tagged yet.
This can also be combined with type to see just a certain type. For example, if I just want to tag Bible Commentaries, I would filter by:
type:bible-commentary ANDNOT mytag:*