[Koha] Empty shelves

[Paul A]

Help requested, please -- the MySQL part of my brain is obviously not at 
the right caffeine level.

Over the years, we have used a lot of "shelves" (authorized values, mostly 
temporary boxes) and now I need to identify the "empty" ones as a 
cleanup/caretaking excercise.

The following SQL query works syntactically, and finds any shelf with at 
least one item:

SELECT authorised_values.authorised_value AS Auth_Location,
COUNT(DISTINCT biblioitems.biblionumber) as Quantity
FROM biblioitems
LEFT JOIN items ON (items.biblioitemnumber=biblioitems.biblioitemnumber)
LEFT JOIN authorised_values ON 
(authorised_values.authorised_value=items.location)
WHERE authorised_values.category LIKE 'loc'
GROUP BY authorised_values.authorised_value HAVING COUNT(DISTINCT 
biblioitems.biblionumber)>0;

but if I change the final >0 to =0 it fails to find any empty box ;={

I'm sort-of convinced that this comes from JOINing the 
authorised_values.authorised_value to the items.location (obviously that 
location no longer exists in items), but am going in circles looking for an 
alternate method.

Thanks in advance -- Paul