Oracle 18c

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CREATE TABLE table_name ( diskno, regdate, regtime, remarks ) AS
SELECT 'd001', '2020/07/01', '12:59:00', 'abc' FROM DUAL UNION ALL
SELECT 'd001', '2020/07/01', '13:00:06', 'def' FROM DUAL UNION ALL
SELECT 'd002', '2020/07/02', '16:00:00', 'ghi' FROM DUAL UNION ALL
SELECT 'd003', '2020/07/04', '07:00:00', 'jkl' FROM DUAL UNION ALL
SELECT 'd003', '2020/07/05', '06:00:50', 'mno' FROM DUAL;
5 rows affected

SELECT diskno,
       MAX( regdate ) as regdate,
       MAX( regtime ) KEEP ( DENSE_RANK LAST ORDER BY regdate, regtime )
         AS regtime,
       MAX( remarks ) KEEP ( DENSE_RANK LAST ORDER BY regdate, regtime )
         AS remarks,
       COUNT(*) AS diskcount
FROM   table_name
GROUP BY diskno


DISKNO
REGDATE
REGTIME
REMARKS
DISKCOUNT


d001
2020/07/01
13:00:06
def
2

d002
2020/07/02
16:00:00
ghi
1

d003
2020/07/05
06:00:50
mno
2


SELECT diskno,
       regdate,
       regtime,
       remarks,
       diskcount
FROM   (
  SELECT t.*,
         ROW_NUMBER() OVER ( PARTITION BY diskno ORDER BY regdate DESC, regtime DESC )
           AS rn,
         COUNT(*) OVER ( PARTITION BY diskno ) AS diskcount
  FROM   table_name t
)
WHERE  rn = 1


DISKNO
REGDATE
REGTIME
REMARKS
DISKCOUNT


d001
2020/07/01
13:00:06
def
2

d002
2020/07/02
16:00:00
ghi
1

d003
2020/07/05
06:00:50
mno
2

DISKNO	REGDATE	REGTIME	REMARKS	DISKCOUNT
d001	2020/07/01	13:00:06	def	2
d002	2020/07/02	16:00:00	ghi	1
d003	2020/07/05	06:00:50	mno	2