Time addition based on previous column partition by different object
SELECT CASE WHEN realtime IS NULL AND @object = object 
            THEN @realtime 
            ELSE @realtime := realtime 
            END realtime,
       realminute,
       @object := object object,
       id,
       @realtime := @realtime + INTERVAL realminute MINUTE nextrealtime
FROM test, (SELECT @object:='',@realtime:='') vars
ORDER BY object, id;


-

| realtime            | object | nextrealtime        | realminute | id  |
| ------------------- | ------ | ------------------- | ---------- | --- |
| 2018-12-12 03:03:03 | A      | 2018-12-12 04:39:03 | 96         | 1   |
| 2018-12-12 04:39:03 | A      | 2018-12-12 04:54:03 | 15         | 2   |
| 2018-12-12 04:54:03 | A      | 2018-12-12 05:04:03 | 10         | 3   |
| 2018-12-12 05:04:03 | A      | 2018-12-12 05:34:03 | 30         | 4   |
| 2018-12-12 05:34:03 | A      | 2018-12-12 06:04:03 | 30         | 5   |
| 2018-12-12 06:04:03 | A      | 2018-12-12 06:19:03 | 15         | 6   |
| 2018-12-12 05:24:01 | B      | 2018-12-12 05:39:01 | 15         | 1   |
| 2018-12-12 05:39:01 | B      | 2018-12-12 05:54:01 | 15         | 2   |
| 2018-12-12 05:54:01 | B      | 2018-12-12 06:09:01 | 15         | 3   |
| 2018-12-12 06:09:01 | B      | 2018-12-12 06:24:01 | 15         | 4   |
| 2018-12-12 06:24:01 | B      | 2018-12-12 06:39:01 | 15         | 5   |
| 2018-12-12 06:39:01 | B      | 2018-12-12 06:54:01 | 15         | 6   |




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