Where do you store DN and any column for Approval?
Here is my shot:
Select aaa.LocationId,bbb.dol_itemnum
from aaa_do aaa
inner join aaa_doline bbb on aaa.do_number = bbb.do_number
inner join aaa_mrline ccc on ccc.mr_number = aaa.mr_number and bbb.dol_itemnum = ccc.mrl_itemnum