code: private void validateOrderItem(SqlSelect select, SqlNode orderItem) {
  		switch (orderItem.getKind()) {
  			case DESCENDING:
  				validateFeature(RESOURCE.sQLConformance_OrderByDesc(),
  					orderItem.getParserPosition());
  				validateOrderItem(select,
  					((SqlCall) orderItem).operand(0));
  				return;
  		}
  
  		final SqlValidatorScope orderScope = getOrderScope(select);
  		validateExpr(orderItem, orderScope);
  	}
docs: Validates an item in the ORDER BY clause of a SELECT statement.
  
  @param select Select statement
  @param orderItem ORDER BY clause item