SQL Oracle - Group by ID, task ID, min and max timestamp

I have data of users performing different tasks. I would like to group this data per userid and task id to get the start and end times per task. When the employee changes to another task, there should be a new row with new start and end time.

Example simplified data set:

example result

I understand that I should work with some min() and max() functions combined with a GROUP BY. However, grouping by userid and taskid, will result in only one row for task-A in this example.

From Oracle 12, you can use MATCH_RECOGNIZE:

SELECT *
FROM   table_name
MATCH_RECOGNIZE(
  PARTITION BY userid
  ORDER BY date_time_stamp
  MEASURES
    FIRST(taskid) AS taskid,
    FIRST(date_time_stamp) AS start_date,
    LAST(date_time_stamp) AS end_date
  ONE ROW PER MATCH
  PATTERN ( same_task+ )
  DEFINE same_task AS FIRST(taskid) = taskid
)

Before that, you can use the ROW_NUMBER analytic function and aggregate:

SELECT userid,
       taskid,
       MIN(date_time_stamp) AS start_date,
       MAX(date_time_stamp) AS end_date
FROM   (
  SELECT t.*,
         ROW_NUMBER() OVER ( PARTITION BY userid ORDER BY date_time_stamp )
           - ROW_NUMBER() OVER ( PARTITION BY userid, taskid ORDER BY date_time_stamp )
           AS grp
  FROM   table_name t
)
GROUP BY userid, taskid, grp
ORDER BY userid, start_date

Which, for your sample data:

CREATE TABLE table_name ( userid, taskid, date_time_stamp ) AS
SELECT 1, 'task-A', DATE '2021-06-16' + INTERVAL '04:17:00' HOUR TO SECOND FROM DUAL UNION ALL
SELECT 1, 'task-A', DATE '2021-06-16' + INTERVAL '04:19:00' HOUR TO SECOND FROM DUAL UNION ALL
SELECT 1, 'task-A', DATE '2021-06-16' + INTERVAL '04:27:00' HOUR TO SECOND FROM DUAL UNION ALL
SELECT 1, 'task-B', DATE '2021-06-16' + INTERVAL '04:31:00' HOUR TO SECOND FROM DUAL UNION ALL
SELECT 1, 'task-B', DATE '2021-06-16' + INTERVAL '04:33:00' HOUR TO SECOND FROM DUAL UNION ALL
SELECT 1, 'task-B', DATE '2021-06-16' + INTERVAL '04:36:00' HOUR TO SECOND FROM DUAL UNION ALL
SELECT 1, 'task-A', DATE '2021-06-16' + INTERVAL '04:42:00' HOUR TO SECOND FROM DUAL UNION ALL
SELECT 1, 'task-A', DATE '2021-06-16' + INTERVAL '04:44:00' HOUR TO SECOND FROM DUAL

Both output:

USERID	TASKID	START_DATE	END_DATE
1	task-A	2021-06-16 04:17:00	2021-06-16 04:27:00
1	task-B	2021-06-16 04:31:00	2021-06-16 04:36:00
1	task-A	2021-06-16 04:42:00	2021-06-16 04:44:00

db<>fiddle here