WITH 
pixel_concernes AS (SELECT bati_indifferencie.id, (ST_DumpAsPolygons(ST_clip(rast, bati_indifferencie.geometrie))).val AS altitude
	FROM ign_bd_topo.mnt34 AS altitude 
	JOIN ign_bd_topo.bati_indifferencie ON st_intersects(rast,geometrie)
	JOIN ign_bd_topo.commune ON st_intersects(commune.geometrie,bati_indifferencie.geometrie)
	WHERE commune.nom = 'Montpellier' 
	)
SELECT id, avg(altitude)
FROM pixel_concernes
GROUP BY id