How to have a conditional SEPARATOR in a GROUP_CONCAT MySQL select - php

Question

I don't know if this can be done or not. In my MySQL code one of my selects is:
GROUP_CONCAT(timestamp SEPARATOR '~') AS times
What I'd like to do is test the timestamps for being equal at the date/hour/min level and if it is replace the SEPARATOR with a <br>
instead of the tilde. Is there a way to do this in MySQL? If not how would I go about doing it in PHP? The end result needs to be the two (or more) timestamps making a string, which will be used in a table cell. If the date/hour/min are equal then both would be in the same cell. If it helps the timestamps would almost certainly be sequential in the table.

Answer 1

You would typically do that with two levels of aggregation. First aggregate by minute and group_concat with the <br> separator, then aggregate the resulting string again with the ~ separator.
Assuming a table called mytable with a timestamp column called mytimestamp, you would do:
select group_concat(gc0 order by myminute separator '~') gc1
from (
select
date_format(mytimestamp, '%Y-%m-%d %H:%i') myminute,
group_concat(mytimestamp order by mytimestamp separator '<br>') gc0
from mytable
group by myminute
) t
Demo on DB Fiddle:
Sample data:
| mytimestamp |
| :------------------ |
| 2019-01-01 00:00:00 |
| 2019-01-01 00:00:01 |
| 2019-01-01 00:00:10 |
| 2019-01-01 00:01:00 |
| 2019-01-02 14:10:00 |
| 2019-01-02 14:10:30 |
Query results:
| gc1 |
| :------------------------------------------------------------------------------------------------------------------------------- |
| 2019-01-01 00:00:00<br>2019-01-01 00:00:01<br>2019-01-01 00:00:10~2019-01-01 00:01:00~2019-01-02 14:10:00<br>2019-01-02 14:10:30 |