表结构

开启两个会话

会话A和会话B

会话A开启事务

会话B开启事务

会话A修改值

update test set value = value + 1 where id = 1;

会话A查询值

select value from test where id = 1;
```sql
![](https://img2020.cnblogs.com/blog/1410909/202108/1410909-20210825183558512-1669303133.png)


### 会话B查询值
```sql
select value from test where id = 1;
```sql
![](https://img2020.cnblogs.com/blog/1410909/202108/1410909-20210825183638953-1458746126.png)

### 会话B修改值
```sql
update test set value = value + 1 where id = 1;

会话B被阻塞

会话A提交事务

会话B修改成功

会话B提交事务

Sql += 时事务问题