DELIMITER //

create function _prev_month ( _date varchar(8) )
returns VARCHAR(6)

begin
   declare ret varchar(6);
   declare pmDate datetime;
	   set pmDate = last_day(date_add( _date, interval -1 month));
	   set ret = concat(substring(pmDate, 1,4),substring(pmDate, 6,2));
   RETURN ret;
end; //

DELIMITER ;

select _prev_month('20220101')