SQL找子元素

DECLARE @tb TABLE
(
    id BIGINT NOT NULL
)
DECLARE @tmp TABLE
(
    id BIGINT NOT NULL
)
DECLARE @tmp2 TABLE
(
    id BIGINT NOT NULL
)
DECLARE @i INT = 0

INSERT @tmp
VALUES
(1)

INSERT @tb
SELECT id
FROM @tmp

WHILE @i < 1000
BEGIN
    INSERT @tmp2
    SELECT main.ID
    FROM UserNodes main
        INNER JOIN @tmp t
            ON main.ParentID = t.id

    IF EXISTS (SELECT 1 FROM @tmp2)
    BEGIN

        DELETE t
        FROM @tmp2 t
            INNER JOIN @tb tb
                ON tb.id = t.id

        INSERT @tb
        SELECT id
        FROM @tmp2

        DELETE FROM @tmp

        INSERT @tmp
        SELECT id
        FROM @tmp2

        DELETE FROM @tmp2
    END
    ELSE
        BREAK
    SET @i = @i + 1
END

SELECT *
FROM @tb