简述MySql四种事务隔离级别

#首先，修改隔离级别
set tx_isolation='READ-UNCOMMITTED';
select @@tx_isolation;
+------------------+
| @@tx_isolation |
+------------------+
| READ-UNCOMMITTED |
+------------------+#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)
在事务B中执行更新语句，且不提交
start transaction;
update tx set num=10 where id=1;
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 10 |
| 2 | 2 |
| 3 | 3 |
+------+------+#事务A：那么这时候事务A能看到这个更新了的数据吗?
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 10 | --->可以看到！说明我们读到了事务B还没有提交的数据
| 2 | 2 |
| 3 | 3 |
+------+------+#事务B：事务B回滚,仍然未提交
rollback;
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+#事务A：在事务A里面看到的也是B没有提交的数据
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 1 |  --->脏读意味着我在这个事务中(A中)，事务B虽然没有提交，但它任何一条数据变化，我都可以看到！
| 2 | 2 |
| 3 | 3 |
+------+------+

#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