sql 触发器例子
```sql
SET ANSI_NULLS ON;
SET QUOTED_IDENTIFIER ON;
GO
ALTER TRIGGER [dnt_user_add] ON [dbo].[dnt_users]
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON;
INSERT INTO [dnt_userfields](uid)
SELECT uid FROM inserted;
END
```
接下来，让我们看看删除操作的触发器。当从[dnt_users]表中删除数据时，[dnt_users_del]触发器就会接管任务，从[dnt_userfields]表中删除与已删除用户对应的记录。代码如下：
```sql
SET ANSI_NULLS ON;
GO
SET QUOTED_IDENTIFIER ON;
GO
CREATE TRIGGER [dnt_users_del] ON [dnt_users]
AFTER DELETE
AS
BEGIN
SET NOCOUNT ON;
DELETE FROM [dnt_userfields] WHERE [dnt_userfields].uid = deleted.uid;
END
GO
```
让我们一下更新操作的触发器。当对[dnt_users]表进行更新操作时，[dnt_users_up]触发器就会被激活。这个触发器的任务是将更新后的用户的uid信息同步到[dnt_userfields]表中。虽然触发器中没有直接的"updated"表，但我们可以通过inserted和deleted表获取更新前后的数据。代码如下：
```sql
SET ANSI_NULLS ON;
GO
SET QUOTED_IDENTIFIER ON;
GO
ALTER TRIGGER [dnt_users_up] ON [dnt_users]
AFTER UPDATE
AS
BEGIN
SET NOCOUNT ON;
UPDATE [dnt_userfields] SET icq = inserted.uid FROM inserted WHERE [dnt_userfields].uid = inserted.uid;
END
GO
```
通过合理利用inserted和deleted这两个临时表，我们可以在数据库的数据发生变动时，实现各种复杂的同步和操作任务。这就是触发器的魅力所在。