I still am confused by DT’s implementation of tags.
Is there no way to do a search for, for example, any file with both the tags “California” and “Case”? Or any boolean search on tags? It seems I have the option to specify that a files tag IS something or IS not something, but not a search that a file’s tags INCLUDE something.
In the Advanced section of the search panel:
Other methods: in Tags view, command-click the two tags to search for records that have both tags. Or, do the same in the Tags group.
(Yes, this all very clunky and so 90s.)
I’m not certain what you mean by “include”.
A hierarchical tag can be created, for example, Author > [Author’s Name] (the second-level tag should be the name of an author, e.g., John Dos Pasos). The rule for hierarchical tags is similar to the rule for hierarchical groups, that is, a group should never include both groups and documents, but only one or the other.
I could populate the Tag “Author” by adding tags that are the names of authors, then populate each of the author name tags with publications by that author.
Now, in the Tags view, if I choose the tag “Author” I will see a list of all the documents that are publications by the various authors in the database. If I click on any one of those documents I will see the tag representing the author’s name. So the tag “Author” includes the names of all the authors (as subtags) and also all the publications (as a list of documents) by any of the authors.
Of course, any of those tagged documents may have been assigned other tags as well. If I click on a document I will see in the Tags Bar all the assigned tags, one of which will be “Author”, because I had, e.g., directly tagged a document as a publication of John Dos Paso, and so indirectly assigned it the Author tag as well.
Can I do a search for those items that have two or more specific tags? Yes, using the Advanced button in the search window, I can do boolean searches for multiple tags predicates (Option click on the “+” button to add additional predicates within the Advanced button editor).
Clunky? I don’t disagree with korm. But then, as I have a somewhat dim view of the return on investment of time and effort expended in tagging (except for special cases as when I’m pulling stuff together for a project), that does’t hurt too much. 
If I were more into tagging, I would be using the Ammonite add-on as an assistant.
Bill,
In your post you say “a group should never include both groups and documents, but only one or the other.” Would you please explain why that is the case? I would appreciate it as I’m trying to expand my understanding of DT. Thanks…
Greg
That recommendation is made by the developers because, for logical reasons, the behavior of Classify and See Also is more effective if a group doesn’t hold both groups and documents.
But this is a rule that, if broken, doesn’t have serious consequences. Breaking the rule means that the group organization isn’t truly hierarchical and the AI assistants will nevertheless work pretty well.
But if you want to create tags that are truly hierarchical, your tagging system won’t be clean unless the tags are assigned only at the lowest appropriate location in the hierarchy.