#首先修改隔离级别
set tx_isolation='read-committed';
select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+

#事务A：启动一个事务
start transaction;
select * from tx;
+------+------+
| id   | num  |
+------+------+
|    1 |    1 |
|    2 |    2 |
|    3 |    3 |
+------+------+

#事务B：也启动一个事务(那么两个事务交叉了)
       在这事务中更新数据，且未提交
start transaction;
update tx set num=10 where id=1;
select * from tx;
+------+------+
| id   | num  |
+------+------+
|    1 |   10 |
|    2 |    2 |
|    3 |    3 |
+------+------+

#事务A：这个时候我们在事务A中能看到数据的变化吗?
select * from tx; --------------->
+------+------+                |
| id   | num  |                |
+------+------+                |
|    1 |    1 |--->并不能看到！  |
|    2 |    2 |                |
|    3 |    3 |                |
+------+------+                |——>相同的select语句，结果却不一样
                               |
#事务B：如果提交了事务B呢?         |
commit;                        |
                               |
#事务A:                         |
select * from tx; --------------->
+------+------+
| id   | num  |
+------+------+
|    1 |   10 |--->因为事务B已经提交了，所以在A中我们看到了数据变化
|    2 |    2 |
|    3 |    3 |
+------+------+

#首先，更改隔离级别
set tx_isolation='repeatable-read';
select @@tx_isolation;
+-----------------+
| @@tx_isolation  |
+-----------------+
| REPEATABLE-READ |
+-----------------+

#事务A：启动一个事务
start transaction;
select * from tx;
+------+------+
| id   | num  |
+------+------+
|    1 |    1 |
|    2 |    2 |
|    3 |    3 |
+------+------+

#事务B：开启一个新事务(那么这两个事务交叉了)
       在事务B中更新数据，并提交
start transaction;
update tx set num=10 where id=1;
select * from tx;
+------+------+
| id   | num  |
+------+------+
|    1 |   10 |
|    2 |    2 |
|    3 |    3 |
+------+------+
commit;

#事务A：这时候即使事务B已经提交了,但A能不能看到数据变化？
select * from tx;
+------+------+
| id   | num  |
+------+------+
|    1 |    1 | --->还是看不到的！(这个级别2不一样，也说明级别3解决了不可重复读问题)
|    2 |    2 |
|    3 |    3 |
+------+------+

#事务A：只有当事务A也提交了，它才能够看到数据变化
commit;
select * from tx;
+------+------+
| id   | num  |
+------+------+
|    1 |   10 |
|    2 |    2 |
|    3 |    3 |
+------+------+

#首先修改隔离界别
set tx_isolation='serializable';
select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| SERIALIZABLE   |
+----------------+

#事务A：开启一个新事务
start transaction;

#事务B：在A没有commit之前，这个交叉事务是不能更改数据的
start transaction;
insert tx values('4','4');
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
update tx set num=10 where id=1;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction