Joining condition for ap_invoices_all and exm_expense_reports

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I am unable to find the approver name when i am doing joining for below query:

select full_name

from

per_person_names_f ppf,

exm_expense_reports eer,

ap_invoices_all ai

where ppf.person_id=eer.current_approver_id

AND ai.reference_key1=eer.expense_report_id;

but i am unable to get the approver name when i am giving joining for ap_invoices table. Without taking invoice table i am getting the approver name.

and is invoice_num and expense_report_num is same?

can you please help me to get the issue resolved.

 

Thanks in advance,