Oracle 18c

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CREATE TABLE table_name ( CAT, NR, "DATE", VERSION, SOME_CODE ) AS
SELECT 'ABC', 	123, 	TIMESTAMP '2009-02-19 00:00:00 UTC', 	1, 	'OPP' FROM DUAL UNION ALL
SELECT 'ABC', 	456, 	TIMESTAMP '2009-03-18 00:00:00 UTC', 	1, 	'ZUM' FROM DUAL UNION ALL
SELECT 'ABC', 	444, 	TIMESTAMP '2009-03-18 00:00:00 UTC', 	1, 	'ZUM' FROM DUAL UNION ALL
SELECT 'ABC', 	444, 	TIMESTAMP '2009-03-18 00:00:00 UTC', 	2, 	'MUZ' FROM DUAL UNION ALL
SELECT 'ABC', 	456, 	TIMESTAMP '2009-04-18 00:00:00 UTC', 	2, 	'XXX' FROM DUAL UNION ALL
SELECT 'ABC', 	456, 	TIMESTAMP '2009-04-18 00:00:00 UTC', 	3, 	'XXX' FROM DUAL UNION ALL
SELECT 'ABC', 	456, 	TIMESTAMP '2009-04-18 00:00:00 UTC', 	4, 	'UIO' FROM DUAL UNION ALL
SELECT 'ABC', 	456, 	TIMESTAMP '2009-05-18 00:00:00 UTC', 	5, 	'RQA' FROM DUAL UNION ALL
SELECT 'DEF', 	637, 	TIMESTAMP '2018-02-16 00:00:00 UTC', 	1, 	'FAW' FROM DUAL UNION ALL
SELECT 'DEF', 	789, 	TIMESTAMP '2018-02-17 00:00:00 UTC', 	1, 	'WER' FROM DUAL UNION ALL
SELECT 'GHI', 	248, 	TIMESTAMP '2018-02-17 00:00:00 UTC', 	1, 	'QWE' FROM DUAL UNION ALL
SELECT 'GHI', 	248, 	TIMESTAMP '2019-02-17 00:00:00 UTC', 	2, 	'PPP' FROM DUAL UNION ALL
SELECT 'GHI', 	357, 	TIMESTAMP '2020-02-16 00:00:00 UTC', 	1, 	'FFF' FROM DUAL UNION ALL
SELECT 'GHI', 	420, 	TIMESTAMP '2020-02-16 00:00:00 UTC', 	1, 	'QDS' FROM DUAL UNION ALL
SELECT 'GHI', 	357, 	TIMESTAMP '2020-02-16 00:00:00 UTC', 	2, 	'GGG' FROM DUAL UNION ALL
SELECT 'GHI', 	357, 	TIMESTAMP '2020-02-16 00:00:00 UTC', 	3, 	'LLL' FROM DUAL UNION ALL
SELECT 'GHI', 	357, 	TIMESTAMP '2020-02-16 00:00:00 UTC', 	4, 	'LLL' FROM DUAL UNION ALL
SELECT 'GHI', 	357, 	TIMESTAMP '2020-08-16 00:00:00 UTC', 	4, 	'FFF' FROM DUAL UNION ALL
SELECT 'GHI', 	357, 	TIMESTAMP '2020-10-16 00:00:00 UTC', 	5, 	'ZZZ' FROM DUAL
19 rows affected

SELECT *
FROM   (
  SELECT t.*,
         TRUNC( "DATE", 'MM' ) AS month
  FROM   table_name t
)
MATCH_RECOGNIZE(
  PARTITION BY cat, month
  ORDER BY "DATE", version
  MEASURES
    MATCH_NUMBER()     AS mn,
    FIRST( some_code ) AS change_from,
    LAST( some_code )  AS change_to
  ONE ROW PER MATCH
  AFTER MATCH SKIP TO LAST change_code
  PATTERN ( strt change_code )
  DEFINE
    change_code AS change_code.some_code <> strt.some_code
)


CAT
MONTH
MN
CHANGE_FROM
CHANGE_TO


ABC
01-MAR-09
1
ZUM
MUZ

ABC
01-APR-09
1
XXX
UIO

DEF
01-FEB-18
1
FAW
WER

GHI
01-FEB-20
1
FFF
QDS

GHI
01-FEB-20
2
QDS
GGG

GHI
01-FEB-20
3
GGG
LLL


SELECT cat,
       month,
       COUNT(*)
FROM   (
  SELECT t.*,
         TRUNC( "DATE", 'MM' ) AS month
  FROM   table_name t
)
MATCH_RECOGNIZE(
  PARTITION BY cat, month
  ORDER BY "DATE", version
  ONE ROW PER MATCH
  AFTER MATCH SKIP TO LAST change_code
  PATTERN ( strt change_code )
  DEFINE
    change_code AS change_code.some_code <> strt.some_code
)
GROUP BY cat, month


CAT
MONTH
COUNT(*)


ABC
01-MAR-09
1

ABC
01-APR-09
1

DEF
01-FEB-18
1

GHI
01-FEB-20
3
CAT	MONTH	MN	CHANGE_FROM	CHANGE_TO
ABC	01-MAR-09	1	ZUM	MUZ
ABC	01-APR-09	1	XXX	UIO
DEF	01-FEB-18	1	FAW	WER
GHI	01-FEB-20	1	FFF	QDS
GHI	01-FEB-20	2	QDS	GGG
GHI	01-FEB-20	3	GGG	LLL