DB2 LUW MERGE using same table to update a different row

My table data looks like this

My poorly attempted SQL is this…

MERGE INTO PRINT target  
USING  
(  
    select ID,PDF_FILE   
    from PRINT  
    where date(PRINTED) = '2022-01-06'  
      and PDF_FILE is not null  
) sause  
ON (target.ID = sause.ID)  
WHEN MATCHED THEN  
    UPDATE SET target.PDF_FILE = sause.PDF_FILE  

It is updating all rows in the table. I do not want this.

How can I make it ONLY update the 1 latest PRINTED row which has an empty PDF_FILE ?

Answer

The idea is to enumerate target rows and update only the 1-st one.

MERGE INTO 
(
SELECT ID, PDF_FILE, ROW_NUMBER () OVER (PARTITION BY ID ORDER BY PRINTED DESC) AS RN_ 
FROM PRINT
WHERE
-- Below is an appropriate condition for
-- the target rows to distinguish them from the source row

-- PDF_FILE IS NULL
date (PRINTED) <> '2022-01-06'
) target
USING
(
select ID,PDF_FILE
from PRINT
where date (PRINTED) = '2022-01-06'
and PDF_FILE is not null
) sause
ON target.ID = sause.ID AND target.RN_ = 1 
WHEN MATCHED THEN
UPDATE SET target.PDF_FILE = sause.PDF_FILE