Uden at se skemaet finder jeg det svært, men dette burde sætte dig i den rigtige retning:
FROM apps.po_requisition_lines_all prl
INNER JOIN apps.po_requisition_headers_all prha ON prl.requisition_header_id = prha.requisition_header_id
INNER JOIN po.po_req_distributions_all prda ON prda.requisition_line_id = prl.requisition_line_id
LEFT JOIN po.po_distributions_all pda ON prda.distribution_id = pda.req_distribution_id
-- I note from the example provided that this is a right join
-- Without seeing the schema, it looks to me as though it should be left
-- As I say say, without seeing the schema, I probably shouldn't pass comment
RIGHT JOIN po.po_headers_all pha ON pha.po_header_id = pda.po_header_id;
For en INNER JOIN du kan bare sige JOIN selvom jeg tror, at udtrykkeligt siger INNER hjælper med at læse. Jeg bemærker også, at eksemplet har WHERE 1=1 hvilket er overflødigt.