MYSQL JSON_MERGE and Group By

Assemble desired JSON using string functions and then cast it into JSON.

Example data

create table data (json json, timestamp datetime);

insert into data values
('{"a": 1}', '2016-10-01 00:00:00'),
('{"b": 2}', '2016-10-01 00:00:00'),
('{"c": 3}', '2016-10-01 11:11:11'),
('{}', '2016-10-01 11:11:11'),
('{"c": 33}', '2016-10-01 11:11:11');

Query to merge all json values grouped by timestamp

select cast(
  concat('{',  -- wrap everything in root object '{ ... }'
    group_concat(
      -- strip parenthesis from individual item representation
      -- '{"foo": 1}' -> '"foo": 1'
      substring(json, 2, length(json) - 2)),
  '}') as json) json,
timestamp
from data
-- skip empty JSON values to avoid getting extra comma during 
-- group_concat
where json != JSON_OBJECT()  
group by timestamp;

Query result

+------------------+---------------------+
| json             | timestamp           |
|------------------+---------------------|
| {"a": 1, "b": 2} | 2016-10-01 00:00:00 |
| {"c": 3}         | 2016-10-01 11:11:11 |
+------------------+---------------------+

Several caveats:

• Behaviour of this snippet is different from JSON_MERGE(), for example:
  • When two or more properties have the same name their values are overwritten instead of being merged into array of values
  • It can't merge objects with arrays
• Solution as presented only works with objects as a top level entity and not with arrays. It can be modified to work with arrays.
• If relies on string representation of JSON objects beginning and ending with curly brackets {}. This might change in future versions of the server.