Get first value or default from json object in T-SQL - sql-server

Question

I have tried this code:
DECLARE #json_doc nvarchar(4000) = '{"Name1":"Value1", "Name2":"Value2"}';
SELECT
CASE WHEN (SELECT COUNT(*) FROM OPENJSON(#json_doc)) > 0
THEN JSON_VALUE(#json_doc,'$.' + (SELECT TOP(1) [key] FROM OPENJSON(#json_doc)))
ELSE NULL END
But it gives me this error:
Msg 13610 Level 16 State 1 Line 3
The argument 2 of the "JSON_VALUE or JSON_QUERY" must be a string literal.
Anyone know how to get this element?

Answer 1

I think you are looking for something like this:
DECLARE #json_doc nvarchar(4000) = '{"Name1":"Value1", "Name2":"Value2"}';
WITH CTE AS
(
SELECT [Value],
-- row number is ordered by the keys first appearence in the json source.
ROW_NUMBER() OVER(ORDER BY CHARINDEX('"' + [Key] +'":', #json_doc)) As rn
FROM OPENJSON(#Json_doc)
)
-- Get the value if json_doc contains any value
SELECT [Value] As ValueOrDefault
FROM CTE
WHERE rn = 1
UNION
-- Get null if not
SELECT NULL
WHERE NOT EXISTS (SELECT 1 FROM CTE);
The result of this query would be Value1 for that json.
However, if the #json_doc would be empty (set #json_doc = '{}';) it will return NULL.
DB<>Fiddle

Answer 2

This should work for you:
DECLARE #json_doc nvarchar(4000) = '{"Name1":"Value1", "Name2":"Value2"}';
DECLARE #sql nvarchar(max) = '
DECLARE #json_doc nvarchar(4000) = ''' + #json_doc + ''';
SELECT
CASE WHEN (SELECT COUNT(*) FROM OPENJSON(#json_doc)) > 0
THEN JSON_VALUE(#json_doc,''$.' + (SELECT TOP(1) [key] FROM OPENJSON(#json_doc)) + ''')
ELSE NULL END
';
EXECUTE (#sql);