distance_from_zone.sql

DROP TABLE qlayer;

SELECT distances.dist, parcels.parcel_id, parcels.geometry INTO qlayer FROM 

  ( SELECT min(ST_Distance(parcels.geometry,firm.geometry)::integer) AS dist, parcels.parcel_id
    FROM parcels,firm
    WHERE firm.zone = 'X500'
    GROUP BY parcels.parcel_id
    ORDER BY dist DESC
  ) AS distances,
  parcels

WHERE parcels.parcel_id = distances.parcel_id;