mysql触发器(插入时更新关联表的某个字段)

数据库表结构

student表

dorm表

1、为student表建立触发器，当向学生表中插入宿舍id时，自动将dorm表该宿舍已住人数加1

create trigger triggerNumber
before insert on student
for each row
begin
 if new.dorm_id is not null
 then
  update dorm set alreadyNumber=alreadyNumber+1 where id=new.dorm_id;
  end if;
end;

2、为student表建立触发器，当删除学生信息时，将关联的宿舍人数减1

create TRIGGER studentDelete
before delete on student
for each row
begin
  if old.dorm_id is not null
  then
   update dorm set alreadyNumber=alreadyNumber-1 where id=old.dorm_id;
   end if;
end;