MySQL 5.5

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create table t(
  arrivalDate varchar(100),
  arrivalTime varchar(100),
  departureDate varchar(100),
  departureTime varchar(100)
);
insert into t values
('20-06-21', '06:00',  NULL,       NULL),
('19-06-21', '08:00', '24-06-21', '08:00'),
(NULL,       NULL,    '20-06-21', '08:00');
SELECT *
FROM (
    SELECT
        STR_TO_DATE(arrivalDate, '%d-%m-%y') AS date,
        CAST(arrivalTime AS TIME) AS arrivalTime,
        NULL AS departureTime
    FROM t
    WHERE arrivalDate IS NOT NULL
    UNION ALL
    SELECT
        STR_TO_DATE(departureDate, '%d-%m-%y'),
        NULL,
        CAST(departureTime AS TIME)
    FROM t
    WHERE departureDate IS NOT NULL
) AS sq1
ORDER BY date, COALESCE(arrivalTime, departureTime)


date
arrivalTime
departureTime


2021-06-19
08:00:00
null

2021-06-20
06:00:00
null

2021-06-20
null
08:00:00

2021-06-24
null
08:00:00

date	arrivalTime	departureTime
2021-06-19	08:00:00	null
2021-06-20	06:00:00	null
2021-06-20	null	08:00:00
2021-06-24	null	08:00:00