Perfect, thanks for that. Now, how many documents don't have images?
select count ( * )  from  ( SELECT document_name FROM documents EXCEPT SELECT t1.document_name FROM documents AS t1 JOIN document_sections AS t2 ON t1.document_code   =   t2.document_code JOIN document_sections_images AS t3 ON t2.section_id   =   t3.section_id ) 
documents without images refers to counting documents that do not join with document_sections_images; EXCEPT is used to find documents not in another set;