> show variables like "char%";

+--------------------------+------------------------------+

| Variable_name            | Value                        |

+--------------------------+------------------------------+

| character_set_client     | utf8mb4                      |

| character_set_connection | utf8mb4                      |

| character_set_database   | utf8mb4                      |

| character_set_filesystem | binary                       |

| character_set_results    | utf8mb4                      |

| character_set_server     | utf8mb4                      |

| character_set_system     | utf8                         |

| character_sets_dir       | /usr/share/mariadb/charsets/ |

+--------------------------+------------------------------+

$ echo -n "ö" | od -c

0000000 303 266

0000002

$ mysql -N -e "SELECT JSON_SET('{}', '$.a', 'ö');" | od -c

0000000   {   "   a   "   :       " 303 266   "   }  \n

0000014

$ mysql -N -e "SELECT JSON_SET('{}', '$.a', 'ö', '$.b', 'ö');" | od -c

0000000   {   "   a   "   :       " 303 266   "   ,       "   b   "   :

0000020       " 366   "   }  \n

0000026

> SELECT JSON_SET('{}', '$.a', 'ö', '$.b', 'ö', '$.c', 1);

+----------------------------------------------------+

| JSON_SET('{}', '$.a', 'ö', '$.b', 'ö', '$.c', 1)   |

+----------------------------------------------------+

| NULL                                               |

+----------------------------------------------------+

1 row in set, 1 warning (0.00 sec)

> show warnings;

+---------+------+------------------------------------------------------------------------+

| Level   | Code | Message                                                                |

+---------+------+------------------------------------------------------------------------+

| Warning | 4035 | Broken JSON string in argument 1 to function 'json_set' at position 16 |

+---------+------+------------------------------------------------------------------------+

1 row in set (0.00 sec)

> SELECT JSON_SET('{}', '$.a', 'ö', '$.c', 1, '$.b', 'ö');

+----------------------------------------------------+

| JSON_SET('{}', '$.a', 'ö', '$.c', 1, '$.b', 'ö')   |

+----------------------------------------------------+

| {"a": "ö", "c": 1, "b": "ö"}                       |

+----------------------------------------------------+

1 row in set (0.00 sec)

$ echo -n "XXXX" | od -c

0000000 360 237 230 253

0000004

$ mysql -N -e "SELECT JSON_SET('{}', '$.a', 'XXXX');" | od -c

0000000   {   "   a   "   :       " 360 237 230 253   "   }  \n

0000016

> SELECT JSON_SET('{}', '$.a', 'XXXX', '$.b', 'XXXX');

+----------------------------------------+

| JSON_SET('{}', '$.a', '?', '$.b', '?') |

+----------------------------------------+

| NULL                                   |

+----------------------------------------+

1 row in set, 1 warning (0.00 sec)

> show warnings;

+---------+------+-------------------------------------------------------------------------------+

| Level   | Code | Message                                                                       |

+---------+------+-------------------------------------------------------------------------------+

| Warning | 4038 | Syntax error in JSON text in argument 1 to function 'json_set' at position 24 |

+---------+------+-------------------------------------------------------------------------------+

1 row in set (0.00 sec)

> SELECT JSON_SET('{}', '$.a', 'XXXX', '$.b', 1, '$.c', 'XXXX');

+--------------------------------------------------+

| JSON_SET('{}', '$.a', '?', '$.b', 1, '$.c', '?') |

+--------------------------------------------------+

| {"a": "XXXX", "b": 1, "c": "XXXX"}                     |

+--------------------------------------------------+

1 row in set (0.00 sec)