Get Dynamic PIVOT result executed with in EXEC() to a varchar variable - json

Question

Below code creates a JSON output.
Note that : The values inside #Person table are generating dynamically. Once I call the stored proc which contains the below code segment, the C# code identifies the output as a JSON eventhough it looks like a VARCHAR.I need the output JSON to be a string(VARCHAR) since the C# model cannot be generated dynamically based on the values returned by #Names.
DROP TABLE #Names
DROP TABLE #PersonInfo
CREATE TABLE #Names (ID INT,Name VARCHAR(MAX))
CREATE TABLE #PersonInfo (ID INT,NameID INT,Subject VARCHAR(100),Marks INT)
INSERT INTO #Names VALUES (1,'Paul');
INSERT INTO #Names VALUES (2,'John');
INSERT INTO #Names VALUES (3,'Tayler');
INSERT INTO #PersonInfo VALUES (1,1,'Maths',95);
INSERT INTO #PersonInfo VALUES (2,2,'Science',32);
INSERT INTO #PersonInfo VALUES (3,3,'History',23);
INSERT INTO #PersonInfo VALUES (4,2,'Maths',32);
INSERT INTO #PersonInfo VALUES (5,3,'Science',60);
INSERT INTO #PersonInfo VALUES (6,1,'Music',60);
DECLARE #DynamicCols NVARCHAR(MAX) = '';
DECLARE #pvt NVARCHAR(MAX) = '';
DECLARE #SQLQuery NVARCHAR(MAX) = '';
SELECT #DynamicCols += ', SUM(' +QUOTENAME([Name])+') AS '+[Name] FROM #Names;
SET #DynamicCols = STUFF(#DynamicCols,1,1,'')
SELECT #pvt += ', ' +QUOTENAME([Name]) FROM #Names;
SET #pvt = STUFF(#pvt,1,1,'')
EXEC ('
SELECT [Subject],' + #DynamicCols+'
FROM (SELECT [NameID], [Subject], [Marks] FROM #PersonInfo) a
INNER JOIN #Names b ON a.NameID = b.ID
PIVOT
(
SUM([Marks])
FOR [Name] IN ('+ #pvt+')
) PIV
GROUP BY [Subject] FOR JSON AUTO, INCLUDE_NULL_VALUES');
Result
[{"Subject":"History","Paul":null,"John":null,"Tayler":23},{"Subject":"Maths","Paul":95,"John":32,"Tayler":null},{"Subject":"Music","Paul":60,"John":null,"Tayler":null},{"Subject":"Science","Paul":null,"John":32,"Tayler":60}]
C# Code in the DAL Layer
public string GetMarks(string partyRoleIdList)
{
return _context.Query<string>(usp_GetMarks, new { IdList }).SingleOrDefault();
}
Is there a way to convert the executed value (EXEC()) to a VARCHAR(MAX) ??

Answer 1

You can easily test that the result of FOR JSON is NVARCHAR(MAX) string:
exec sp_describe_first_result_set N'
SELECT *
FROM
(
VALUES(''[a'')
) DS(col)
FOR JSON AUTO
';
Yields this:
is_hidden column_ordinal name is_nullable system_type_id system_type_name max_length precision scale collation_name
0 1 JSON_F52E2B61-18A1-11d1-B105-00805F49916B 1 231 nvarchar(max) -1 0 0 SQL_Latin1_General_CP1_CI_AS
And it is string, so you can cast it to VARCHAR(MAX) or something else easily:
EXEC ('
SELECT CAST(
(
SELECT [Subject],' + #DynamicCols+'
FROM (SELECT [NameID], [Subject], [Marks] FROM #PersonInfo) a
INNER JOIN #Names b ON a.NameID = b.ID
PIVOT
(
SUM([Marks])
FOR [Name] IN ('+ #pvt+')
) PIV
GROUP BY [Subject] FOR JSON AUTO, INCLUDE_NULL_VALUES)
AS VARCHAR(400))');