code: function (cols) {
              var ret;
              if (!this.hasSelectSource) {
                  ret = this.select.apply(this, arguments);
              } else {
                  ret = this.mergeOptions();
              }
              return ret;
          }
docs: Selects the columns if only if there is not already select sources.
  
  @example
  
  var ds = DB.from("items"); //SELECT * FROM items
  
  ds.select("a"); //SELECT a FROM items;
  ds.select("a").selectIfNoSource("a", "b"). //SELECT a FROM items;
  ds.selectIfNoSource("a", "b"). //SELECT a, b FROM items;
  
  @param cols columns to select if there is not already select sources.
  @return {patio.Dataset} a cloned dataset with the appropriate select sources.