calculates the percentiles of col_name 
        WHERE [where_col_list] = [where_value_list]

def calc_percentiles(self, col_name, where_col_list, where_value_list):
        """ 
        calculates the percentiles of col_name 
        WHERE [where_col_list] = [where_value_list]
        """
        #col_data = self.get_col_data_by_name(col_name)
        col_data = self.select_where(where_col_list, where_value_list, col_name)
        #print('calc_percentiles: col_data = ', col_data, ' where_col_list = ', where_col_list, ', where_value_list = ', where_value_list)
        if len(col_data) == 0:
            #print("Nothing to calculate")
            return 0,0,0
        else:
            first  = self.percentile(col_data, .25)
            third  = self.percentile(col_data, .75)
            median = self.percentile(col_data, .50)
            #print('CALC_PERCENTILES =  first, third, median ',  first, third, median )
            return  first, third, median