SQL Server 2019

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CREATE TABLE #Primary(PrimaryId int,ItemName varchar(10))
CREATE TABLE #Sub1(PrimaryId int,SubId int,SubName1 varchar(10))
CREATE TABLE #Sub2(PrimaryId int,Sub2Id int,SubName2 varchar(10))


INSERT INTO #Primary
VALUES(1,'My Item1'),(2,'My Item2')

INSERT INTO #Sub1 values(1,1,'Sub1'),(1,2,'Sub2')

INSERT INTO #Sub2 values(1,1,'Name1'),(1,2,'Name2')
6 rows affected

SELECT P.PrimaryID,
       P.ItemName,
       (SELECT S1.SubId,
               S1.SubName1
        FROM #Sub1 S1
        WHERE S1.PrimaryId = P.PrimaryId
        FOR JSON AUTO) AS sb1,
       (SELECT S2.Sub2Id,
               S2.SubName2
        FROM #Sub2 S2
        WHERE S2.PrimaryId = P.PrimaryId
        FOR JSON AUTO) AS sb2
FROM #Primary P
WHERE P.PrimaryID = 1
FOR JSON AUTO;


JSON_F52E2B61-18A1-11d1-B105-00805F49916B


[{"PrimaryID":1,"ItemName":"My Item1","sb1":[{"SubId":1,"SubName1":"Sub1"},{"SubId":2,"SubName1":"Sub2"}],"sb2":[{"Sub2Id":1,"SubName2":"Name1"},{"Sub2Id":2,"SubName2":"Name2"}]}]


DROP TABLE #Primary;
DROP TABLE #Sub1;
DROP TABLE #Sub2;