CREATE TABLE t1 (c VARCHAR(8));

INSERT INTO t1 VALUES ('abc'),('def');

SELECT

  JSON_OBJECT('foo', JSON_UNQUOTE(JSON_OBJECT('bar', c)),'qux', c) AS fld

FROM t1;

# Cleanup

DROP TABLE t1;

MariaDB [test]> SELECT

    ->   JSON_OBJECT('foo', JSON_UNQUOTE(JSON_OBJECT('bar', c)),'qux', c) AS fld

    -> FROM t1;

ERROR 1271 (HY000): Illegal mix of collations for operation 'json_object'

MySQL [test]> SELECT

    ->   JSON_OBJECT('foo', JSON_UNQUOTE(JSON_OBJECT('bar', c)),'qux', c) AS fld

    -> FROM t1;

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

| fld                                         |

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

| {"foo": "{\"bar\": \"abc\"}", "qux": "abc"} |

| {"foo": "{\"bar\": \"def\"}", "qux": "def"} |

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

2 rows in set (0.00 sec)