設置MySQL事務隔離級別
select @@tx_isolation; //查看隔離級別
set transaction isolation level read uncommitted; //設置讀未提交級別
start transaction; //打開事務
rollback; //回滾
commit; //提交
set transaction isolation level read committed; //設置讀提交級別
set transaction isolation level repeatable read; //設置可重復讀（缺省），保證每次讀的結果是一樣的
set transaction isolation level serializable; //設置成序列化
