The implementation uses / symbol as a nesting separator. I.e. tag/subtag is a nested tag, where tag is the parent tag and subtag is its child. Creating a tag named tag/subtag/subsubtag creates three tags, one for each level. The tags are associated using parent_id field.
In the app, viewing notes with a tag will also show all notes that are associated with any of the tag's descendant tags (same for the note count). Deleting a tag will also delete all its descendant tags.
In the desktop app the tags are shown nested just like the notebooks.
