多張表的刪除—觸發器實現

在web開發時，經常會有多張表的刪除操作。例如，師生互動平臺，教務處部分，刪除學生的時候，許多其他表中也會有studentid這個字段，那麼，如果單純的刪除basic_info_student表中的該學生，下一次，許多表在遍歷的時候，就會出現，找不到學生基本信息的情況。此時，解決辦法是：1，把所有的有studentid字段的表中該記錄刪除。2，將studentid更新成爲另外一個默認值。我們這裏採用的是第二種，當然，第一種和第二種原理相同。

1，新建刪除觸發器

create or replace trigger studelete
after DELETE
ON basic_info_student
for each row
BEGIN delete from ELECOURSE_INFO_USER where Vstudentid=:old.Vstudentid;
END studelete;

2，在從basic_info_student表中刪除數據的時候，就會級聯刪除數據

delete from basic_info_student where Vstudentid='1'

3，最後的版本

create or replace trigger studelete
after DELETE
ON basic_info_student
for each row
BEGIN delete from ELECOURSE_INFO_USER where Vstudentid=:old.Vstudentid;
update agreestudent set Vstudentid='0000001' where Vstudentid=:old.Vstudentid;
delete from apply_list where Vstudentid=:old.Vstudentid;
delete from discuss_App where Vstudentid=:old.Vstudentid;
delete from DM_ask where studentid=:old.Vstudentid;
delete from Dm_bbs where studentid=:old.Vstudentid;
delete from Dm_discuss where studentid=:old.Vstudentid;
delete from Dm_dminfo where studentid=:old.Vstudentid;
delete from Dm_interact where studentid=:old.Vstudentid;
delete from Dm_logon where studentid=:old.Vstudentid;
delete from Dm_qsearch where studentid=:old.Vstudentid;
delete from ELECOURSE_INFO_USER where Vstudentid=:old.Vstudentid;
update label_lesson set Vstudentid='0000001' where Vstudentid=:old.Vstudentid;
update Posts set Vpersonid='0000001' where Vpersonid=:old.Vstudentid and Ipersontype='11';
delete from ptcourse_info_user where Vstudentid=:old.Vstudentid;
update question_list set Vstudentid='0000001' where Vstudentid=:old.Vstudentid;
update Replypost set Vpersonid='0000001' where Vpersonid=:old.Vstudentid and Ipersontype='11';
update sms_list set Vsenderid='0000001' where Vsenderid=:old.Vstudentid and ISenderType='11';
delete from student_file where Vstudentid=:old.Vstudentid;
delete from student_sms where Vstudentid=:old.Vstudentid;
END studelete;

問題：

1，什麼是觸發器。

2，但是級聯刪除數據庫的時候，如果數據庫裏面字段對應的有文件id，如何刪除附件呢？

3，如何編寫事物。