mysql 触发器问题

有2个表 T1（id  title  number）  T2（id tid title）  t2表中tid外键关联t1表id

现在我想在t2表上建立个触发器，在执行 插入 和删除 操作后， 更新t1表的对应行的number （number为t2的总数 ti。id=t2.tid，相当于一个帖子和这个帖子的回复那种关系）


请问这个触发器要什么写啊？

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SQL code:

create trigger t2_delete_trig after delete on t2
for each row
update t1 set number=number-1 where id=old.tid;

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SQL code:
create trigger tr_t2_ai after insert on t2
for each row
update t1 set `number` =`number` +1 where id=new.tid;
create trigger tr_t2_ad after delete on t2
for each row
update t1 set `number` =`number` -1 where id=old.tid;


SQL code:
mysql> create trigger tr_t2_ai after insert on t2
-> for each row
-> update t1 set `number` =`number` +1 where id=new.tid;
Query OK, 0 rows affected (0.19 sec)

mysql> create trigger tr_t2_ad after delete on t2
-> for each row
-> update t1 set `number` =`number` -1 where id=old.tid;
Query OK, 0 rows affected (0.16 sec)

mysql> select * from t1;
+------+-------