title: How to return JSON_ARRAY using an GROUP_CONCAT
body: <p>I've try return json object using "concat" and "group_concat" functions.
  Problem is that I need to use group_concat but I want to have a valid JSON structure. What I do wrong?</p>
  
  <ol>
  <li>{"a":"a", "b": "b", "id": null}</li>
  <li>[{"id": "123"}]</li>
  </ol>
  
  <p>...</p>
  
  <pre><code>SELECT JSON_REPLACE((
                    SELECT JSON_OBJECT(
                               'a', 'a',
                               'b', 'b',
                               'id', null
                             )), '$.id', (
                    SELECT CONCAT(
                               '[', group_concat(JSON_OBJECT(
                            'id',
                            '123')),
                               ']'))
       )
  </code></pre>
  
  <p>result: <code>{"a": "a", "b": "b", "id": "[{\"id\": \"123\"}]"}</code></p>
  
  <p>expected: <code>{"a": "a", "b": "b", "id": [{"id": "123"}]}</code></p>