SELECT
   verbale
 , tipo_verbale AS tipover
 , COALESCE(id_progetto, 'N/D') AS idprogetto
 , COALESCE(programma, 'N/D') AS programma
 , COALESCE(linea, 'N/D') AS linea
 , COALESCE(linea, 'N/D') AS clin
FROM verbali
NATURAL JOIN progetti
WHERE COALESCE(tipo_verbale = :tipover, TRUE)
  AND COALESCE(id_progetto = :idprogetto, TRUE)
  AND COALESCE(programma = :programma, TRUE)
  AND COALESCE(linea = :linea, TRUE)
