SQL Server 2022

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CREATE TABLE Table1( 
  acctnum int,
  note varchar(max),
  srno int 
)
INSERT INTO Table1 (acctnum, note, srno)
VALUES(12, 'abc', 1),
(null, 'efd', 2),
(null, 'xyz', 3),
(15, 'pqr', 4),
(16, 'uio', 5),
(null, 'njk', 6),
(null, 'uuuwr', 7),
(null, 'kmjkj', 8),
(18, 'zzzzzzz', 9)
9 rows affected

;WITH CTE AS
(
  SELECT 1 Lvl, acctnum, note, srno, note newnote
  FROM Table1
  WHERE acctnum IS NOT NULL
  UNION ALL
  SELECT C.Lvl +1, C.acctnum, T.note, T.srno, CONCAT(C.newnote, ' ', T.note) newnote
  FROM CTE C
  INNER JOIN Table1 T
    ON (T.srno - C.srno = 1)
      AND T.acctnum IS NULL
),
FINAL AS
(
  SELECT *, ROW_NUMBER() OVER(PARTITION BY acctnum ORDER BY Lvl DESC) RN
  FROM CTE
)
SELECT acctnum, newnote note
FROM FINAL
WHERE RN =1


acctnum
note


12
abc efd xyz

15
pqr

16
uio njk uuuwr kmjkj

18
zzzzzzz