SELECT  res.familiya as res_familiya,
                res.imya as res_imya,
                res.otchestvo as res_otchestvo,
                res.state_payment as res_state_payment,
                res.published as res_published,

                res.razmeshenie as res_razmeshenie,
                res.mob_telefon as res_mob_telefon,
                res.email as res_email,

                res.id as res_id,
                
                (SELECT GROUP_CONCAT(date_field) FROM #__reservation_dates WHERE id_reservation = res.id) as dates
                
                
        FROM #__reservation AS res 
        
        ORDER BY res.id DESC LIMIT 20