Sql server 2005 update触发器语句

当self_id列填充下一条记录时，如何在自动触发的触发器中使用以下查询

UPDATE  tree_str
SET     L1_Left = ml.self_id,
    L1_Centre = mc.self_id,
    L1_Right = mr.self_id
FROM    tree_str t LEFT JOIN
    member ml   ON  t.self_id = ml.parent_id
                AND ml.position = 'LEFT' LEFT JOIN
    member mc   ON  t.self_id = mc.parent_id
                AND mc.position = 'CENTER' LEFT JOIN
    member mr   ON  t.self_id = mr.parent_id
                AND mr.position = 'RIGHT'

我使用下面的触发器，但它不工作

CREATE TRIGGER autoupdate ON [dbo].[tree_str]
FOR UPDATE
AS BEGIN

UPDATE  tree_str
SET     L1_Left = ml.self_id,
    L1_Centre = mc.self_id,
    L1_Right = mr.self_id
FROM    tree_str t LEFT JOIN
    member ml   ON  t.self_id = ml.parent_id
                AND ml.position = 'LEFT' LEFT JOIN
    member mc   ON  t.self_id = mc.parent_id
                AND mc.position = 'CENTER' LEFT JOIN
    member mr   ON  t.self_id = mr.parent_id
                AND mr.position = 'RIGHT'
END                    

---

请尝试以下触发器语法：

CREATE TRIGGER autoupdate ON dbo.tree_str
AFTER UPDATE
AS BEGIN
    UPDATE  tree_str
    SET L1_Left = ml.self_id, L1_Centre = mc.self_id,  L1_Right = mr.self_id
    FROM tree_str t LEFT JOIN
    member ml   ON  t.self_id = ml.parent_id AND ml.position = 'LEFT' 
    LEFT JOIN member mc   ON  t.self_id = mc.parent_id AND mc.position = 'CENTER' 
    LEFT JOIN member mr   ON  t.self_id = mr.parent_id
    AND mr.position = 'RIGHT'

END

---

一个明显的变化是“更新后”而不是“更新前”。

请至少尝试自己编写触发器。。。。