You need to create recursive SQL function like below :
CREATE FUNCTION getSubNodes (@SectionId INT)
RETURNS VARCHAR(MAX)
AS
BEGIN
DECLARE @SubNodes VARCHAR(MAX)
SET @SubNodes = ''
IF EXISTS(SELECT SectionId FROM Section_T WHERE SectionParentId = @SectionId)
BEGIN
SELECT @SubNodes = COALESCE(@SubNodes + '','') +
S_Start + dbo.getSubNodes(SectionId) + S_End
FROM Section_T
WHERE SectionParentId = @SectionId
ORDER BY SectionOrder
END
RETURN @SubNodes
END
and you can use that function like below :
SELECT S_Start + dbo.getSubNodes(SectionId) + S_End
FROM Section_T
WHERE SectionParentId IS NULL