Return the lag/lead function with the offset and default value

def get_field_identifier(self):
        """
        Return the lag/lead function with the offset and default value
        """
        if self.default is None:
            return '{0}, {1}'.format(self.field.get_select_sql(), self.offset)
        return "{0}, {1}, '{2}'".format(self.field.get_select_sql(), self.offset, self.default)