MariaDB 10.2

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CREATE TABLE table1
    (id int, time timestamp);
INSERT INTO table1
    (id, time)
VALUES
    (1, '2018-05-15 09:00'),
    (1, '2018-05-15 09:10'),
    (1, '2018-05-15 09:20'),
    (2, '2018-05-15 09:30'),
    (2, '2018-05-15 09:40');
CREATE TABLE table2
    (id int, time timestamp);
INSERT INTO table2
    (id, time)
VALUES
    (1, '2018-05-15 09:21'),
    (1, '2018-05-15 09:31'),
    (2, '2018-05-15 09:41');
WITH
  t1 AS
  ( SELECT id, time, 
           row_number() OVER (PARTITION BY id ORDER BY time DESC) AS rn
    FROM table1
  ), 
  t2 AS
  ( SELECT id, time, 
           row_number() OVER (PARTITION BY id ORDER BY time DESC) AS rn
    FROM table2
  )
SELECT 
    t1.id, 
    CASE WHEN t2.time IS NULL OR t1.time >= t2.time
        THEN t1.time ELSE t2.time
    END AS time,
    CASE WHEN t2.time IS NULL OR t1.time >= t2.time
        THEN 'table1' ELSE 'table2'
    END AS "table"
FROM
    t1 LEFT JOIN t2 
    ON  t1.id = t2.id
    AND t1.rn = t2.rn
ORDER BY
    id, t1.rn DESC ;


id
time
table


1
2018-05-15 09:00:00
table1

1
2018-05-15 09:21:00
table2

1
2018-05-15 09:31:00
table2

2
2018-05-15 09:30:00
table1

2
2018-05-15 09:41:00
table2

id	time	table
1	2018-05-15 09:00:00	table1
1	2018-05-15 09:21:00	table2
1	2018-05-15 09:31:00	table2
2	2018-05-15 09:30:00	table1
2	2018-05-15 09:41:00	table2