Extracting value using common_schema not working for nested JSON - mysql

Question

I have a DB field called data of type is MEDIUMTEXT, which stores values in JSON format. I'm using extract_json_value method from common_schema.
When JSON is not nested, it works fine. For example, when applications_data table's data field is
{
"key": "value"
}
This query works fine:
SELECT data into #json from applications_data;
SELECT common_schema.extract_json_value(#json, 'key') as result;
and gives result: key
However, when the data field is a nested JSON, it fails. For example, JSON is:
{
"key": {
"overview": "sample"
}
}
Using the same query as above and result is empty, not NULL:

Answer 1

Remember:
extract_json_value
...
This function internally relies on json_to_xml(): it first
converts the JSON data to XML, then uses ExtractValue to apply
XPath.
...
and
ExtractValue(xml_frag, xpath_expr)
...
If no matching text node is found for the expression (including the
implicit /text())—for whatever reason, as long as xpath_expr is valid, and xml_frag consists of elements which are properly nested and closed—an empty string is returned. No distinction is made between a match on an empty element and no match at all. This is by design.
If you need to determine whether no matching element was found in
xml_frag or such an element was found but contained no child text
nodes, you should test the result of an expression that uses the XPath
count() function.
...
Test:
mysql> SET #`json` := '
'> {
'> "key": {
'> "overview": "sample"
'> }
'> }
'> ';
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT
-> common_schema.extract_json_value(#`json`, 'key') AS result0,
-> common_schema.extract_json_value(#`json`, count('key')) AS result1,
-> common_schema.extract_json_value(#`json`, 'key/overview') AS result2,
-> common_schema.extract_json_value(#`json`, count('key/overview')) AS result3;
+---------+---------+---------+---------+
| result0 | result1 | result2 | result3 |
+---------+---------+---------+---------+
| | 1 | sample | 1 |
+---------+---------+---------+---------+
1 row in set (0.03 sec)