Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (Dept, Employee, TransactionDate, Event) AS
SELECT 'dept1', 'emp1', DATE '2022-05-20', 'abgd' FROM DUAL UNION ALL
SELECT 'dept1', 'emp1', DATE '2022-05-20', 'ggg' FROM DUAL UNION ALL
SELECT 'dept1', 'emp1', DATE '2022-05-20', 'hdfh' FROM DUAL UNION ALL
SELECT 'dept2', 'emp2', DATE '2022-01-26', '3fdfds' FROM DUAL UNION ALL
SELECT 'dept2', 'emp2', DATE '2022-01-26', 'dsfsd' FROM DUAL UNION ALL
SELECT 'dept2', 'emp2', DATE '2022-01-26', '554fsds' FROM DUAL UNION ALL
SELECT 'dept2', 'emp2', DATE '2022-01-26', 'gg32' FROM DUAL UNION ALL
SELECT 'dept2', 'emp2', DATE '2022-01-26', 'fd4gfg' FROM DUAL;
8 rows affected

SELECT t.*,
       COUNT(*) OVER (PARTITION BY Dept, Employee, TransactionDate) AS cnt
FROM   table_name t


DEPT
EMPLOYEE
TRANSACTIONDATE
EVENT
CNT


dept1
emp1
2022-05-20 00:00:00
abgd
3

dept1
emp1
2022-05-20 00:00:00
hdfh
3

dept1
emp1
2022-05-20 00:00:00
ggg
3

dept2
emp2
2022-01-26 00:00:00
gg32
5

dept2
emp2
2022-01-26 00:00:00
554fsds
5

dept2
emp2
2022-01-26 00:00:00
dsfsd
5

dept2
emp2
2022-01-26 00:00:00
fd4gfg
5

dept2
emp2
2022-01-26 00:00:00
3fdfds
5

DEPT	EMPLOYEE	TRANSACTIONDATE	EVENT	CNT
dept1	emp1	2022-05-20 00:00:00	abgd	3
dept1	emp1	2022-05-20 00:00:00	hdfh	3
dept1	emp1	2022-05-20 00:00:00	ggg	3
dept2	emp2	2022-01-26 00:00:00	gg32	5
dept2	emp2	2022-01-26 00:00:00	554fsds	5
dept2	emp2	2022-01-26 00:00:00	dsfsd	5
dept2	emp2	2022-01-26 00:00:00	fd4gfg	5
dept2	emp2	2022-01-26 00:00:00	3fdfds	5