使用JSON_SET（）更新JSON doc的部分内容，而不是将整个doc设置为new

I want to update parts of a JSON doc inside a MySQL database table.

The field name in the database: jdata

The Problem

I do not want to set it like jdata = '{_whatever_}', because I would loose keys.

I need to preserve existing keys (e.g. key1, key3) and only set key2 to its new designated value 999.

The JSON doc

{
  "calculation":{
    "key1": 100,
    "key2": 200,
    "key3": 300
  }
}

The PHP/MySQL

$a = json_encode(['key2' => 999], JSON_UNESCAPED_SLASHES);
$sql =
"   UPDATE  svtr.auftrag
    SET     jdata = JSON_SET(jdata, '$.calculation', '{$a}')
    WHERE[...]
";

Using the full path like so: JSON_SET(jdata, '$.calculation.key2', 999) is no option here, since the original JSON doc is much more complex and I would have to iterate many keys. Think of it like having 30 key/value pairs and needing to update 29 of them.

Hope anyone can make sense of my ramblings ... Thanks in advance!