1.事务隔离级别：
一共4种
read uncommitted
read committed
repeatable read
serializable
关于各种隔离级别的理解：
https://www.cnblogs.com/rjzheng/p/9955395.html
https://baijiahao.baidu.com/s?id=1611918898724887602&wfr=spider&for=pc
自己的理解：
read uncommitted: 事务a读取到了事务b未提交的改动，脏读
read committed：事务a读取到了事务b提交后的改动
repeatable read：事务a从头到尾查询结果一致，即便事务b做了改动甚至提交了。
serializable：
查看隔离级别的命令
select @@tx_isolation;
修改隔离级别
set session transaction isolation level (read uncommitted / read committed / repeatable read / serialized read )