Group by sequel date

I do not exactly know which table you are using but here is an example.

Table data:

Select statement to find interval of intervals:

SELECT company_id, emp_no,date_of_employment,
       (SELECT MAX(date_of_leaving)
        FROM emp_employed_time_row t
        CONNECT BY t.company_id = PRIOR t.company_id
        AND t.emp_no = PRIOR t.emp_no
        AND t.date_of_employment = CASE WHEN PRIOR t.date_of_leaving = to_date('9999-12-31','YYYY-MM-DD') THEN PRIOR t.date_of_leaving ELSE PRIOR t.date_of_leaving + 1 END
        START WITH t.company_id = e.company_id
        AND t.emp_no = e.emp_no
        AND t.date_of_employment = e.date_of_employment) date_of_leaving
FROM emp_employed_time_row e
WHERE company_id = '300'
AND NOT EXISTS (SELECT 1
                FROM emp_employed_time_row x
                WHERE x.company_id = e.company_id
                AND x.emp_no = e.emp_no
                AND e.date_of_employment -1= x.date_of_leaving)

Short explanation:

The main select fetches all start of intervals you want. Then the end date is fetched by a query in the SELECT part.

I had to do special handling of date 9999-21-31 due to the data I had. Not needed if you do not have/will have that kind of data.