SQL Server 2017

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select * into Departments 
from(
  values
     (1, cast('/' as hierarchyid),	 'MHz'),
     (2, '/2/',	 'Finance'),
     (3, '/3/',	 'IT'),
     (4, '/3/4/', 'Software Development'),
     (5, '/3/5/', 'QA'),
     (6, '/2/6/', 'Accountant')

) t(Id,	HierarchyId, Name);
6 rows affected

CREATE FUNCTION dbo.GetDepartmentAsJson(@departmentId hierarchyid, @level int)
RETURNS nvarchar(max)
AS
BEGIN
DECLARE @Json NVARCHAR(MAX) = '{}',
    @Id int,
    @Name varchar(50),
    @Childeren NVARCHAR(MAX) = '{}',
    @Hierar Hierarchyid

    SET @Json = (SELECT
    t.Id,
    t.HierarchyId,
    t.Name,
    children = JSON_QUERY(dbo.GetDepartmentAsJson(t.HierarchyId, @level + 1))
    FROM Departments t
    WHERE t.HierarchyId <> @departmentId
    AND t.HierarchyId.IsDescendantOf(@departmentId) = 1 and t.HierarchyId.GetLevel() =  @level + 1
    FOR JSON PATH);

    IF(@level = 0) 
    BEGIN
        SELECT
        @Id = t.Id,
        @Hierar = t.HierarchyId,
        @Name = t.Name
        FROM Departments t
        WHERE t.HierarchyId = @departmentId;

        SET @Json = 
        '{"Id":"' + CONVERT(varchar(7), @Id) +
        '","HierarchyId":"' + @Hierar.ToString() + 
        '","Name":"' + @Name + 
        '","Children":' + CAST(@Json AS NVARCHAR(MAX)) + '}';
    END

    return @Json;

  END;
select dbo.GetDepartmentAsJson ('/', 0)


(No column name)


{"Id":"1","HierarchyId":"/","Name":"MHz","Children":[{"Id":2,"HierarchyId":"\/2\/","Name":"Finance","children":[{"Id":6,"HierarchyId":"\/2\/6\/","Name":"Accountant"}]},{"Id":3,"HierarchyId":"\/3\/","Name":"IT","children":[{"Id":4,"HierarchyId":"\/3\/4\/","Name":"Software Development"},{"Id":5,"HierarchyId":"\/3\/5\/","Name":"QA"}]}]}