TSQL: Recursive Descent in JSON Path - json

Question

I was hoping to take advantage of $.. in json_value() function within TSQL queries using stored json documents. Unfortunately it doesn't work:
JSON path is not properly formatted. Unexpected character '.' is found at position 2.
and according to documentation, there was even no intention to:
Path steps can contain the following elements and operators.
Key names. For example, $.name and $."first name". If the key name starts with a dollar sign or contains special characters such as spaces, surround it with quotes.
Array elements. For example, $.product[3]. Arrays are zero-based.
The dot operator (.) indicates a member of an object. For example, in $.people1.surname, surname is a child of people.
Is there any other method, how to find the attribute a at any level(s) in structured json stored in a TSQL table column?
For example, let's have a on arbitrary level in the json document:
select json_value(json, '$..a') from data would return both values (thus 1, 2) for following values in data.json column:
first:
{
"a": 1
}
second:
{
"b": {
"a": 2
}
}

Answer 1

SQL Server has indeed limited support for JSON expressions.
You could mix a recursive common table expression with the openjson() function.
Sample data
create table data
(
json nvarchar(max)
);
insert into data (json) values
('{ "a": 1 }'),
('{ "b": { "a": 2 } }');
Solution
with rcte as
(
select x.[key] as path,
x.[key],
x.[value],
x.[type]
from data d
cross apply openjson(d.json, '$') x
union all
select r.[path] + '.' + rr.[key],
rr.[key],
rr.[value],
rr.[type]
from rcte r
cross apply openjson(r.[value]) rr
where r.[type] = 5 -- 5=object
)
select r.[key],
r.[value],
r.[path]
from rcte r
where r.[key] = 'a';
Result
key value path
--- ----- ----
a 1 a
a 2 b.a
Fiddle (with intermediate recursive common table expression results).