generate SQL to insert staging table records into
        the core table based on column_list (If no column list
        then insert sequentially)

def populate_from_staging(self, staging_table, from_column_list, output_table):
        """ 
        generate SQL to insert staging table records into
        the core table based on column_list (If no column list
        then insert sequentially)
        """
        self.sql_text += 'INSERT INTO ' + output_table + ' (\n'
        for c in self.col_list:
            if c != '':
                self.sql_text += '    ' + c + ',\n'
        self.sql_text += '    ' + self.date_updated_col + ') (\n'
        self.sql_text += '    SELECT \n'
        for c in from_column_list:
            if c != '':
                self.sql_text += '        ' + c + ',\n'
        self.sql_text += '        SYSDATE \n    FROM ' + staging_table
        self.sql_text += '\n);\n'