HCM Extract | Aggregate Function in value set

Hi ,

Requirement ->This value set returns minimum assignment effective start date for the given person and date range coming from fast formula( Extract rule type).

I used this logic, which looks good to me , inner query is returning the data but outside not returning the data . Could you please let me know where am I doing wrong?

select n.n

from (select level n from dual connect by level <= 1000000)n

where n.n = (select a.val from    

       (SELECT 

 to_number(to_char(MIN(EFFECTIVE_START_DATE), 'yyyymmdd')) val

       FROM

       PER_ALL_ASSIGNMENTS_M  

       WHERE 

       assignment_type='E' 

       and system_person_type='EMP'