cartodbv2.js

cartodb.createLayer(map, 'viz.json').done(function(layer) {
   map.addLayer(layer);

   var countries = layer.getSubLayer(0);
   countries.setSQL('select * from countries');

   var places = layer.createSubLayer({
      sql: "select * from populated_places",
      cartocss: "#layer { marker-fill: red; }"
   })

   setTimeout(function() {
     places.remove();
   }, 10000);

});


// creating a map without viz.json

cartodb.createLayer(map, {
   type: 'cartodb',
   layers: [{
     sql: '....',
     cartocss: '....'
   }]
}).done(function(layer) {
  leaflet.addLayer(layer)
});