MySQL 8.0

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create table t(
    id int not null primary key auto_increment,
    data varchar(200)
)
insert into t(data) values
('{}'),
('{"new_key":null}')

Records: 2  Duplicates: 0  Warnings: 0


update t set data = json_insert(data, '$.new_key', json_object(
    'key1', json_object(
        'key11', 'value11',
        'key12', 'value12'
    ),
    'key2', json_object(
        'key21', 'value21',
        'key22', 'value22'
    )
))

Rows matched: 2  Changed: 2  Warnings: 0


select *
from t


id
data


1
{"new_key": {"key1": {"key11": "value11", "key12": "value12"}, "key2": {"key21": "value21", "key22": "value22"}}}

2
{"new_key": null}

id	data
1	{"new_key": {"key1": {"key11": "value11", "key12": "value12"}, "key2": {"key21": "value21", "key22": "value22"}}}
2	{"new_key": null}