How to query the database for files having one tag but missing another?

I have minimal experience with elisp, and absolutely zero experience with SQL queries. My goal is to tweak one of the functions d12frosted shared here so that it checks not only for the presence of the project tag, but for also the absence of the excluded tag (which I’m using to exclude irrelevant files from the agenda). Here’s the original function, which satisfies only the first of my two conditions:

(defun vulpea-project-files ()
  "Return a list of note files containing 'project' tag." ;
     [:select [nodes:file]
      :from tags
      :left-join nodes
      :on (= tags:node-id nodes:id)
      :where (like tag (quote "%\"project\"%"))]))))

Thanks in advance for any help.