code: function(oSelectedPropertyNames) {
  			if (this._aSortCondition.length == 0) {
  				return "";
  			}
  
  			var sOrderByOptionString = "";
  			for (var i = -1, oCondition; (oCondition = this._aSortCondition[++i]) !== undefined;) {
  				if (!oSelectedPropertyNames[oCondition.property.name]) {
  					continue; // sorting of aggregated entities is meaningful only if the sorted property is also selected
  				}
  				sOrderByOptionString += (sOrderByOptionString == "" ? "" : ",") + oCondition.property.name + " " + oCondition.order;
  			}
  
  			return sOrderByOptionString;
  		}
docs: Get the value for the OData system query option $orderby corresponding to
  this expression.
  
  @param {object} oSelectedPropertyNames Object with properties requested for $select
  
  @returns {string} The $orderby value for the sort expressions
  @public
  @function
  @name sap.ui.model.analytics.odata4analytics.SortExpression#getURIOrderByOptionValue