This is an example on how to compute overlaps on a timeline.

timeline_overlapping.sql

-- 1. T_INTERVALS:
--     [-----------------------------[ #1
--   08:00                         08:30
--               [---------[ #2
--             08:10    08:20
--                                             [-------------------[ #3
--                                           08:40               09:00
--                                        [----------[ #4
--                                      08:35      08:45
--                                                            [---------[ #5
--                                                          08:55     09:05
--                                                                      [---------[ #6
--                                                                    09:05     09:15
--
-- 2. SQL Query: 
--     [-----------------------------[
--   08:00                         08:30
--                                        [---------------------------------------[
--                                      08:35                                   09:15
--
CREATE TABLE
  t_intervals (
    begin_time DATE, 
    end_time DATE, 
    category VARCHAR2(5), 
    code VARCHAR2(5)
  );

INSERT INTO t_intervals(begin_time, end_time, category, code) VALUES (to_date('08:00', 'HH24:MI'), to_date('08:30', 'HH24:MI'), 'A', '#1');
INSERT INTO t_intervals(begin_time, end_time, category, code) VALUES (to_date('08:10', 'HH24:MI'), to_date('08:20', 'HH24:MI'), 'A', '#2');
INSERT INTO t_intervals(begin_time, end_time, category, code) VALUES (to_date('08:40', 'HH24:MI'), to_date('09:00', 'HH24:MI'), 'A', '#3');
INSERT INTO t_intervals(begin_time, end_time, category, code) VALUES (to_date('08:35', 'HH24:MI'), to_date('08:45', 'HH24:MI'), 'A', '#4');
INSERT INTO t_intervals(begin_time, end_time, category, code) VALUES (to_date('08:55', 'HH24:MI'), to_date('09:05', 'HH24:MI'), 'A', '#5');
INSERT INTO t_intervals(begin_time, end_time, category, code) VALUES (to_date('09:05', 'HH24:MI'), to_date('09:15', 'HH24:MI'), 'A', '#6');
INSERT /*+ APPEND */ INTO t_intervals(begin_time, end_time, category, code) SELECT i.begin_time + 10 / (24 * 60) begin_time, i.end_time + 10 / (24 * 60) end_time, 'B', i.code FROM t_intervals i;
COMMIT;

SELECT 
  i.category,
  LISTAGG(i.code, ', ') WITHIN GROUP(ORDER BY i.begin_time, i.code) codes, 
  MIN(i.begin_time) begin_time,
  MAX(i.end_time) end_time
FROM
  (
    SELECT
      i.category,
      i.code, 
      i.begin_time,
      i.end_time,
      MAX(i.group_1) OVER(PARTITION BY category ORDER BY i.begin_time ASC) group_2
    FROM
      (
        SELECT 
          i.category,
          i.begin_time,
          i.code, 
          COALESCE(i.end_time, to_date('99991230', 'YYYYMMDD')) end_time,
          CASE
            WHEN 
                  (i.begin_time NOT BETWEEN MIN(i.begin_time) OVER(PARTITION BY i.category ORDER BY i.begin_time ASC ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING) AND COALESCE(MAX(i.end_time) OVER(PARTITION BY i.category ORDER BY i.begin_time ASC ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING), to_date('99991230', 'YYYYMMDD')))  
               OR LAG(i.begin_time) OVER(PARTITION BY category ORDER BY i.begin_time ASC) IS NULL 
            THEN
              row_number() OVER(PARTITION BY i.category ORDER BY i.begin_time ASC)
            ELSE
              -1
          END group_1
        FROM
          t_intervals i
      ) i
  ) i
GROUP BY
  i.category,
  i.group_2;

DROP TABLE 
  t_intervals;