set transaction isolation level read uncommitted; //设置读未提交级别
start transaction; //打开事务
rollback; //回滚
commit; //提交
set transaction isolation level read committed; //设置读提交级别
set transaction isolation level repeatable read; //设置可重复读（缺省），保证每次读的结果是一样的
set transaction isolation level serializable; //设置成序列化