MariaDB starting with 10.5.0
----------------------------
JSON_OBJECTAGG was added in MariaDB 10.5.0.

Syntax
------

JSON_OBJECTAGG(key, value)

Description
-----------

JSON_OBJECTAGG returns a JSON object containing key-value pairs. It takes two
expressions that evaluate to a single value, or two column names, as
arguments, the first used as a key, and the second as a value.

The maximum returned length in bytes is determined by the group_concat_max_len
server system variable.

Returns NULL in the case of an error, or if the result contains no rows.

JSON_OBJECTAGG cannot currently be used as a window function.

Examples
--------

select * from t1;
+------+-------+
| a    | b     |
+------+-------+
|    1 | Hello |
|    1 | World |
|    2 | This  |
+------+-------+

SELECT JSON_OBJECTAGG(a, b) FROM t1;
+----------------------------------------+
| JSON_OBJECTAGG(a, b)                   |
+----------------------------------------+
| {"1":"Hello", "1":"World", "2":"This"} |
+----------------------------------------+

URL: https://mariadb.com/kb/en/json_objectagg/