#首先，修改隔離級(jí)別
set tx_isolation='READ-UNCOMMITTED';
select @@tx_isolation;
+------------------+
| @@tx_isolation |
+------------------+
| READ-UNCOMMITTED |
+------------------+#事務(wù)A：?jiǎn)?dòng)一個(gè)事務(wù)
start transaction;
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+#事務(wù)B：也啟動(dòng)一個(gè)事務(wù)(那么兩個(gè)事務(wù)交叉了)
在事務(wù)B中執(zhí)行更新語句，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 |
+------+------+#事務(wù)A：那么這時(shí)候事務(wù)A能看到這個(gè)更新了的數(shù)據(jù)嗎?
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 10 | --->可以看到！說明我們讀到了事務(wù)B還沒有提交的數(shù)據(jù)
| 2 | 2 |
| 3 | 3 |
+------+------+#事務(wù)B：事務(wù)B回滾,仍然未提交
rollback;
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+#事務(wù)A：在事務(wù)A里面看到的也是B沒有提交的數(shù)據(jù)
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 1 |  --->臟讀意味著我在這個(gè)事務(wù)中(A中)，事務(wù)B雖然沒有提交，但它任何一條數(shù)據(jù)變化，我都可以看到！
| 2 | 2 |
| 3 | 3 |
+------+------+

#首先修改隔離級(jí)別
set tx_isolation='read-committed';
select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+#事務(wù)A：?jiǎn)?dòng)一個(gè)事務(wù)
start transaction;
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+#事務(wù)B：也啟動(dòng)一個(gè)事務(wù)(那么兩個(gè)事務(wù)交叉了)
在這事務(wù)中更新數(shù)據(jù)，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 |
+------+------+#事務(wù)A：這個(gè)時(shí)候我們?cè)谑聞?wù)A中能看到數(shù)據(jù)的變化嗎?
select * from tx; --------------->
+------+------+    |
| id | num |    |
+------+------+    |
| 1 | 1 |--->并不能看到！ |
| 2 | 2 |    |
| 3 | 3 |    |
+------+------+    |——>相同的select語句，結(jié)果卻不一樣
|
#事務(wù)B：如果提交了事務(wù)B呢?   |
commit;      |
|
#事務(wù)A:       |
select * from tx; --------------->
+------+------+
| id | num |
+------+------+
| 1 | 10 |--->因?yàn)槭聞?wù)B已經(jīng)提交了，所以在A中我們看到了數(shù)據(jù)變化
| 2 | 2 |
| 3 | 3 |
+------+------+

#首先，更改隔離級(jí)別
set tx_isolation='repeatable-read';
select @@tx_isolation;
+-----------------+
| @@tx_isolation |
+-----------------+
| REPEATABLE-READ |
+-----------------+#事務(wù)A：?jiǎn)?dòng)一個(gè)事務(wù)
start transaction;
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+------+------+#事務(wù)B：開啟一個(gè)新事務(wù)(那么這兩個(gè)事務(wù)交叉了)
在事務(wù)B中更新數(shù)據(jù)，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;#事務(wù)A：這時(shí)候即使事務(wù)B已經(jīng)提交了,但A能不能看到數(shù)據(jù)變化？
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 1 | --->還是看不到的！(這個(gè)級(jí)別2不一樣，也說明級(jí)別3解決了不可重復(fù)讀問題)
| 2 | 2 |
| 3 | 3 |
+------+------+#事務(wù)A：只有當(dāng)事務(wù)A也提交了，它才能夠看到數(shù)據(jù)變化
commit;
select * from tx;
+------+------+
| id | num |
+------+------+
| 1 | 10 |
| 2 | 2 |
| 3 | 3 |
+------+------+

#首先修改隔離界別
set tx_isolation='serializable';
select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| SERIALIZABLE |
+----------------+#事務(wù)A：開啟一個(gè)新事務(wù)
start transaction;#事務(wù)B：在A沒有commit之前，這個(gè)交叉事務(wù)是不能更改數(shù)據(jù)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