Basic CartoDB Layer in D3

index.js

d3.json("http://account_name.cartodb.com/api/v2/sql?q=SELECT * FROM table_name WHERE the_geom IS NOT NULL&format=geojson&dp=5", function(collection) { 
  svg.select("#layer_id")
    .selectAll("path")
    .data(collection.features)
    .enter().append("path")
    .attr("fill", "violet")
    .attr("d", path.projection(xy));
});