session B：
mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> update tb_book set book_name = '多情刀客无情刀' where book_id = 1; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> rollback; Query OK, 0 rows affected (0.00 sec)结果：事务A读到了事务B还没提交的中间状态，即产生了脏读。
不可重复读(read committed)事务A读到了事务B已经提交的修改数据
操作：
session A事务隔离级别设置为read committed并开启事务，首次查询book_id为1的记录；
然后session B开启事务，并修改book_id为1的记录，不提交事务，在session A中再次查询book_id为1的记录；
最后提交session B中的事务，再在session A中查看book_id为1的记录。
session A：
mysql> set session transaction isolation level read committed; Query OK, 0 rows affected (0.01 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tb_book where book_id = 1; +---------+-----------------------+--------+ | book_id | book_name | author | +---------+-----------------------+--------+ | 1 | 多情剑客无情剑 | 古龙 | +---------+-----------------------+--------+ 1 row in set (0.00 sec) mysql> select * from tb_book where book_id = 1; +---------+-----------------------+--------+ | book_id | book_name | author | +---------+-----------------------+--------+ | 1 | 多情剑客无情剑 | 古龙 | +---------+-----------------------+--------+ 1 row in set (0.00 sec) mysql> select * from tb_book where book_id = 1; +---------+-----------------------+--------+ | book_id | book_name | author | +---------+-----------------------+--------+ | 1 | 多情刀客无情刀 | 古龙 | +---------+-----------------------+--------+ 1 row in set (0.00 sec) mysql> commit; Query OK, 0 rows affected (0.00 sec)session B：
mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> update tb_book set book_name = '多情刀客无情刀' where book_id = 1; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> commit; Query OK, 0 rows affected (0.00 sec)结果：事务B没有提交事务时，事务A不会读到事务B修改的中间状态，即read committed解决了上面所说的脏读问题，但是当事务B中的事务提交后，事务A读到了修改后的记录，而对于事务A来说，仅仅读了两次，却读到了两个不同的结果，违背了事务之间的隔离性，所以说该事务隔离级别下产生了不可重复读的问题。
幻读(repeatable read)事务A读到了事务B提交的新增数据
操作：
session A事务隔离级别设置为repeatable read并开启事务，并查询book列表
session B开启事务，先修改book_id为5的记录，再插入一条新的数据，提交事务，在session A中再次查询book列表
在session A中更新session B中新插入的那条数据，再查询book列表
session A：
mysql> set session transaction isolation level repeatable read; Query OK, 0 rows affected (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tb_book; +---------+-----------------------+--------+ | book_id | book_name | author | +---------+-----------------------+--------+ | 1 | 多情刀客无情刀 | 古龙 | | 2 | 笑傲江湖 | 金庸 | | 3 | 倚天屠龙记 | 金庸 | | 4 | 射雕英雄传 | 金庸 | | 5 | 绝代双骄 | 古龙 | +---------+-----------------------+--------+ 5 rows in set (0.00 sec) mysql> select * from tb_book; +---------+-----------------------+--------+ | book_id | book_name | author | +---------+-----------------------+--------+ | 1 | 多情刀客无情刀 | 古龙 | | 2 | 笑傲江湖 | 金庸 | | 3 | 倚天屠龙记 | 金庸 | | 4 | 射雕英雄传 | 金庸 | | 5 | 绝代双骄 | 古龙 | +---------+-----------------------+--------+ 5 rows in set (0.00 sec) mysql> update tb_book set book_name = '圆月弯剑' where book_id = 6; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from tb_book; +---------+-----------------------+--------+ | book_id | book_name | author | +---------+-----------------------+--------+ | 1 | 多情刀客无情刀 | 古龙 | | 2 | 笑傲江湖 | 金庸 | | 3 | 倚天屠龙记 | 金庸 | | 4 | 射雕英雄传 | 金庸 | | 5 | 绝代双骄 | 古龙 | | 6 | 圆月弯剑 | 古龙 | +---------+-----------------------+--------+ 6 rows in set (0.00 sec) mysql> rollback; Query OK, 0 rows affected (0.00 sec)