SQL Query - Case Statement with Null use Max

I have 2 tables, "Payable" and "Payable_Line" which contains detail information.  I want a query to use max if one of the field in my query is "Null".  However, I can't get it to work correctly.

Query output result:
Payble and Payable_line
SELECT P.VOUCHER_ID, P.INVOICE_ID, P.TOTAL_AMT,
              CASE WHEN PL.PO_ID IS NULL THEN MAX(PL.PO_ID) ELSE PL.PO_ID END PO_ID
FROM PAYABLE P LEFT OUTER JOIN PAYABLE_LINE PL
              ON P.VOUCHER_ID = PL.VOUCHER_ID
GROUP BY P.VOUCHER_ID, P.INVOICE_ID, P.TOTAL_AMT, PL.PO_ID

Am I doing something wrong?  Basically wanting to input my max value in the "Null" field for PO_ID.
holemaniaAsked:
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sdstuberCommented:
If you want both rows returned, then try an analytic instead of aggregate

SELECT p.voucher_id,
       p.invoice_id,
       p.total_amt,
       CASE
           WHEN pl.po_id IS NULL
           THEN
               MAX(pl.po_id) OVER (PARTITION BY p.voucher_id, p.invoice_id, p.total_amt)
           ELSE
               pl.po_id
       END
           po_id
  FROM payable p LEFT OUTER JOIN payable_line pl ON p.voucher_id = pl.voucher_id
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holemaniaAuthor Commented:
Awesome.  That's what I was looking for.
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Query Syntax

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