gistfile1.sql

WITH clusters as (
		SELECT 
			cartodb_id,  
			st_snaptogrid(the_geom_webmercator, CDB_XYZ_Resolution({z})) as center 
		FROM table_2007_2009_bicy 
	),
	points as (
		SELECT 
			count(cartodb_id) as npoints, 
			count(cartodb_id)/power( CDB_XYZ_Resolution({z}), 2 ) as density 
		FROM 
			clusters 
		group by 
			center
	),
	stats as (
		SELECT 
			npoints, 
			density, 
			ntile(5) over (order by density) as quartile 
		FROM points 
	) 
SELECT 
	quartile, 
	max(npoints) as maxAmount, 
	max(density) as maxDensity  
FROM stats 

GROUP BY quartile ORDER BY quartile