SELECT 1 ID and all belonging elements - sql

Question

I try to create a json select query which can give me back the result on next way.
1 row contains 1 main_message_id and belonging messages. (Like the bottom image.) The json format is not a requirement, if its work with other methods, it will be fine.
I store the data as like this:
+-----------------+---------+----------------+
| main_message_id | message | sub_message_id |
+-----------------+---------+----------------+
| 1 | test 1 | 1 |
| 1 | test 2 | 2 |
| 1 | test 3 | 3 |
| 2 | test 4 | 4 |
| 2 | test 5 | 5 |
| 3 | test 6 | 6 |
+-----------------+---------+----------------+
I would like to create a query, which give me back the data as like this:
+-----------------+-----------------------+--+
| main_message_id | message | |
+-----------------+-----------------------+--+
| 1 | {test1}{test2}{test3} | |
| 2 | {test4}{test5}{test6} | |
| 3 | {test7}{test8}{test9} | |
+-----------------+-----------------------+--+

Answer 1

You can use json_agg() for that:
select main_message_id, json_agg(message) as messages
from the_table
group by main_message_id;
Note that {test1}{test2}{test3} is invalid JSON, the above will return a valid JSON array e.g. ["test1", "test2", "test3"]
If you just want a comma separated list, use string_agg();
select main_message_id, string_ag(message, ', ') as messages
from the_table
group by main_message_id;