from https://github.com/hotosm/HDM-CartoCSS/blob/bde531/hdm.mml#L949

places.sql

(SELECT 
    way,
    CASE 
      WHEN place='country' THEN 1 
      WHEN capital='yes' AND admin_level='2' THEN 10 
      WHEN place='state' THEN 20 
      WHEN place='city' THEN 21 
      WHEN place='town' THEN 30 
      ELSE NULL 
    END as prio,
    place AS type,
    name,
    z_order,
    CASE 
      WHEN population ~ E'^\\\\d+$' THEN population::integer 
      ELSE NULL
    END AS population, 
    CASE
      WHEN capital='yes' THEN admin_level
      ELSE NULL
    END AS admin_level
  FROM planet_osm_point 
  WHERE 
    place IN ('country', 'state', 'city', 'town') 
  ORDER BY prio, population DESC NULLS LAST) 
AS data