Calculate the difference between this record's value and the lag/lead record's value

def get_select_sql(self):
        """
        Calculate the difference between this record's value and the lag/lead record's value
        """
        return '(({0}) - ({1}({2}){3}))'.format(
            self.field.get_select_sql(),
            self.name.upper(),
            self.get_field_identifier(),
            self.get_over(),
        )