Select
hotel.hotel_id,
hotel.offdb,
hotel.offgg,
hotel.offtexten,
camere.cam_nome,
camere.cam_nome_en,
camere.cam_leg_en,
camere.cam_leg_it,
camere.cam_occ_img,
camere.cam_occ,
Avg(dispo.disp_prezzo) AS avera,
Sum(dispo.disp_prezzo) AS totprezzo,
dispo.disp_occ,
Count(dispo.disp_avail) AS countg,
dispo.disp_avail,
camere.cam_desc,
camere.cam_id
From
dispo
Inner Join camere ON dispo.disp_camera_id = camere.cam_id 
Inner Join hotel ON dispo.disp_hotel_id = hotel_id

Where
dispo.disp_date Between '--0' AND '---1' AND dispo.disp_hotel_id = ''

Group By
dispo.disp_camera_id

Having
dispo.disp_occ = '' 
AND
MIN(dispo.disp_avail) >= ''

Order by
avera ASC