-- example 1

-- ========== 0 =========
start transaction;
-- A先查询
select *
from customer;
-- ========== 1 =========

-- ========== 2 =========
-- 在 B 添加数据后，A 尝试进行修改。奇怪的是，明明查询不到，居然可以修改成功
update customer
set username = 'wuhu'
where id = 1;
-- A 再次查询表中数据，发现修改的那一行现身了
select *
from customer;
commit;
-- ========== 3 =========


-- example 2
-- ========== 0 =========
start transaction;
-- A先查询，查到数据如下：
-- 1,Fortern
-- 2,Maxin
-- 3,MooGeo
select *
from customer;
-- ========== 1 =========

-- ========== 2 =========
-- A 备份表中数据到另一个表
insert into customer_back select * from customer;
commit;
-- 结果备份表中的数据居然是被其他事物修改的数据！！
select * from customer_back;
-- 为什么会这样？
-- ========== 3 =========

select * from customer where id = 1;