the value of this binding is the basename of org-roam (e.g. "roam")

the format +org-roam-node-display-template would fail is the that name exists in a previous directory:

assume the org-roam dir is "/Users/dmg/roam/org/roam/"

(if (string-match
     (format "/%s/\\([^/]+\\)/" "roam")
     "/Users/dmg/roam/org/roam/refs/faq_git.org")
    (match-string 1 "/Users/dmg/roam/org/roam/refs/faq_git.org"))

would return org instead of refs

use the full path instead.

cl-loop is a macro, and I suspect that it uses more memory than mapcar. See the following code:

#+begin_src emacs-lisp   :exports both
(cl-loop for row in (list (list 1 2) (list 2 3))
         collect (apply '+ row))
#+end_src

#+RESULTS:
| 3 | 5 |

#+begin_src emacs-lisp   :exports both
(mapcar (lambda (row)
          (apply '+ row)) (list (list 1 2) (list 2 3)))
#+end_src

#+RESULTS:
| 3 | 5 |

I would also avoid append when possible. mutation in general is bad :) and append (in theory) is less efficient than car.

don't silently remove slots without a mapping

in "cl-remove-if 'null".

raise an error instead. If a column does not exist, it is most likely an error.

YOu can write this query:

 "select %s
		  from nodes_view inner join files on nodes_view.file = files.file
		   %s
		   %s;"
		 column-names

as

 "select %s
		  from nodes_view join files using (file)
		   %s
		   %s;"
		 column-names

I find this much more readable. but it is your choice.

(propertize (org-roam-node-title node) 'face 'bold))))

would it no better to define a face for this purpose instead of hardcoding it?