Oracle 11g Release 2

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create table t as
     SELECT 'A001' as activity_id, to_date('01/01/2020 09:00:00', 'DD/MM/YYYY HH24:MI:SS') as event_timestamp, 'STATUS A' as event_status from dual union all
     SELECT 'A001' as activity_id, to_date('01/01/2020 10:10:00', 'DD/MM/YYYY HH24:MI:SS') as event_timestamp, 'STATUS B' from dual union all
     SELECT 'A001' as activity_id, to_date('01/01/2020 11:20:00', 'DD/MM/YYYY HH24:MI:SS') as event_timestamp, 'STATUS C' from dual union all
     SELECT 'A001' as activity_id, to_date('01/01/2020 12:30:00', 'DD/MM/YYYY HH24:MI:SS') as event_timestamp, 'STATUS C' from dual union all
     SELECT 'A001' as activity_id, to_date('01/01/2020 13:30:00', 'DD/MM/YYYY HH24:MI:SS') as event_timestamp, 'STATUS A' from dual union all
     SELECT 'A002' as activity_id, to_date('01/01/2020 13:40:00', 'DD/MM/YYYY HH24:MI:SS') as event_timestamp, 'STATUS F' from dual union all
     SELECT 'A002' as activity_id, to_date('01/01/2020 17:50:00', 'DD/MM/YYYY HH24:MI:SS') as event_timestamp, 'STATUS F' from dual union all
     SELECT 'A002' as activity_id, to_date('01/01/2020 17:53:00', 'DD/MM/YYYY HH24:MI:SS') as event_timestamp, 'STATUS G' from dual

8 rows affected

ACTIVITY_ID	EVENT_TIMESTAMP	EVENT_STATUS	PREV_EVENT_STATUS	EVENT_NUMBER	EVENT_STATUS_GROUP
A001	01-JAN-20	STATUS A	null	1	1
A001	01-JAN-20	STATUS B	STATUS A	2	2
A001	01-JAN-20	STATUS C	STATUS B	3	3
A001	01-JAN-20	STATUS C	STATUS C	4	3
A001	01-JAN-20	STATUS A	STATUS C	5	4
A002	01-JAN-20	STATUS F	null	1	1
A002	01-JAN-20	STATUS F	STATUS F	2	1
A002	01-JAN-20	STATUS G	STATUS F	3	2