1. An error
ERROR 1241 (21000): Operand should contain 1 column(s)
2. The reason for the error
This statement occurs mostly because the result set of a SELECT is wrapped in (). ()code> s>t in pare>select * from

select 
 pit_key
,employee_code
,department_id
,value_date
from pit_employee_department ped
where ped.employee_code = 'GSCQ3349'
and ped.value_date < date_format(date_sub(curdate(), interval day(curdate()) - 1 day),'%Y%m%d')
and ped.pit_key not in
(	select 
	pit_key
	,value_date
	from pit_employee_department ped_1
	inner join 
	(
		select 
		max(value_date) as max_date
		from pit_employee_department ped

		where ped.value_date <= date_format(date_sub( date_sub(curdate(), interval day(curdate()) - 1 day),interval 1 month),'%Y%m%d')
		and employee_code = 'GSSH0039'
	)ped_2
	on ped_1.value_date < ped_2.max_date
	and ped_1.employee_code = 'GSSH0039'
);

pit_key not in (...) pit_keyd v>_date pit_key not in (...) , field inconsistency causes error.
3. Solutions
Make changes for different reasons.