group-by in cartocss

project.mml

{
  "bounds": [
    13.2644,
    52.4914,
    13.2994,
    52.5071
  ],
  "center": [
    13.2811,
    52.4992,
    15
  ],
  "format": "png8",
  "interactivity": {
    "layer": "avusroad",
    "template_teaser": "fid: {{{fid}}} <br>\ntype: {{{type}}} <br>\ntunnel: {{{tunnel}}} <br>\nbridge: {{{bridge}}} <br>\noneway: {{{oneway}}} <br>\nclass: {{{class}}} <br>\nz_order: {{{z_order}}} <br>\nz: {{{z}}} <br>",
    "fields": [
      "fid",
      "type",
      "tunnel",
      "bridge",
      "oneway",
      "class",
      "z_order",
      "z"
    ]
  },
  "minzoom": 0,
  "maxzoom": 22,
  "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
  "Stylesheet": [
    "style.mss"
  ],
  "Layer": [
    {
      "id": "countries",
      "name": "countries",
      "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
      "geometry": "polygon",
      "Datasource": {
        "file": "http://mapbox-geodata.s3.amazonaws.com/natural-earth-1.4.0/cultural/10m-admin-0-countries.zip",
        "type": "shape"
      }
    },
    {
      "geometry": "linestring",
      "extent": [
        13.253320370821777,
        52.47756086705049,
        13.320082039843436,
        52.52463336915364
      ],
      "Datasource": {
        "type": "sqlite",
        "file": "avus.sqlite",
        "table": "(SELECT fid, geometry, type, tunnel, bridge, oneway, class, z_order, CAST((z_order / 10.0) AS INTEGER) AS z FROM roads ORDER BY z_order) AS road",
        "attachdb": "",
        "extent": "",
        "id": "avusroad",
        "project": "avus",
        "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over"
      },
      "properties": {
        "group-by": "z"
      },
      "id": "avusroad",
      "class": "",
      "srs-name": "900913",
      "srs": "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over",
      "name": "avusroad"
    }
  ],
  "scale": 1,
  "metatile": 2,
  "name": "",
  "description": "",
  "legend": "",
  "attribution": ""
}