SQL: ring buffers with several distances

README.md

Query to produce a series of rings using buffers:

• First you get a generate_series with the number of rings you want to produce
• Then produce the 80km buffers multiplying by the radius for a number of cities
• Finally use the lag function to remove for every ring the previous one so you get the ring

buffer.sql

WITH rings AS (
  SELECT generate_series(1, 5) as radius
), circles AS (
  SELECT 
  cartodb_id, 
  radius, 
  ST_Transform(
    ST_Buffer(
      the_geom::geography, 
      radius * 80000
    )::geometry
    ,3857
  ) AS the_geom_webmercator
  FROM populated_places, rings 
  WHERE NAME in
  	('Madrid','Helsinki','Warsaw',
     'London','Trondheim')
)
SELECT
  cartodb_id,
  radius,
  CASE 
  WHEN radius = 1 
    THEN the_geom_webmercator
  ELSE
    ST_Difference(
      the_geom_webmercator,
      lag(the_geom_webmercator) OVER (PARTITION BY cartodb_id ORDER BY radius)
    ) 
  END AS the_geom_webmercator
FROM circles

style.css

/** choropleth visualization */

@color0: #fef6b5;
@color1: #ffd08f;
@color2: #ffa679;
@color3: #f67c78;
@color4: #e15383;

#populated_places{
  polygon-fill: #FFFFB2;
  polygon-opacity: 0.8;
  line-color: #FFF;
  line-width: 2.5;
  line-opacity: 1;
}
#populated_places [ radius = 5] {
   polygon-fill: @color4
}
#populated_places [ radius = 4] {
   polygon-fill: @color3
}
#populated_places [ radius = 3] {
   polygon-fill: @color2
}
#populated_places [ radius = 2] {
   polygon-fill: @color1
}
#populated_places [ radius = 1] {
   polygon-fill: @color0
}