一、
1、
#事务
一个或一组SQL要么全部执行，要么全部不执行
#事务的ACID属性
#原子性（Atomicity）
一个事务是一个整体 不可拆分 要么都成功 要么都失败
#一致性（Consistency）
事务必须使数据库从一个一致性状态，变换到另一个一致性状态
也就是本来两个人金额总和为200，转账后也得是200
#隔离性（Isolation）
两个事务相互隔离，并发执行的各个事务不能相互干扰
#持久性（Durability）
一个事务一但被提交，它对数据库中的数据的改变是永久性的
#start transaction; 开启事务 update user set balance = balance-100 where name = 'hby'; update user set balance = balance-100 where name = 'pdun'; #rollback; #全部撤销（回滚） #commit; 注意：commit前可以回滚，一旦执行了commit，将无法回滚 ------------------------------------------------------------------------ #start transaction; update user set balance = balance-100 where name = 'hby'; update user set balance = balance+100 where name = 'pdun'; #savepoint p; #创建保存点，可以选择回滚到此处，注意：一定取名字 update user set balance = balance+100 where name = 'pdun'; #rollback to p;
#脏读，幻读，不可重复读都有 mysql> select @@tx_isolation; #查看隔离级别 +-----------------+ | @@tx_isolation | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) #设置隔离级别set session transaction isolation level mysql> set session transaction isolation level read uncommitted; #读不提交 mysql> select * from user; +----+-------+---------+ | id | name | balance | +----+-------+---------+ | 1 | pdun | 1000 | | 2 | Moker | 1000 | +----+-------+---------+ 2 rows in set (0.00 sec) mysql> start transaction; #开启事务， Query OK, 0 rows affected (0.00 sec) #修改但未commit mysql> update user set balance = balance+100 where name = 'pdun'; ------------------------------------------- #此时打开另一个终端2，同样把隔离基别设置为最轻等级 mysql> start transaction; #开启事务， mysql> select * from user; +----+-------+---------+ | id | name | balance | +----+-------+---------+ | 1 | pdun | 1100 | #发现数据已经改了，但是终端1并未提交，这就是脏读 | 2 | Moker | 1000 | +----+-------+---------+ #如果终端1 rollback，回滚了 mysql> select * from user; +----+-------+---------+ | id | name | balance | +----+-------+---------+ | 1 | pdun | 1000 | #数据又变回来了，这就称为不可重复读和幻读 | 2 | Moker | 1000 | +----+-------+---------+ 2 rows in set (0.00 sec)
#没有脏读，存在幻读与不可重复读 #开启终端1 #设置隔离级别 mysql> set session transaction isolation level read committed; mysql> select *from user; +----+-------+---------+ | id | name | balance | +----+-------+---------+ | 1 | pdun | 1000 | | 2 | Moker | 1000 | +----+-------+---------+ 2 rows in set (0.00 sec) mysql> start transaction; #开启事务 Query OK, 0 rows affected (0.00 sec) mysql> update user set balance = balance+100 where name = 'Moker'; #没有commit提交 ---------------------------------- #开启终端2 mysql> select * from user; +----+-------+---------+ | id | name | balance | +----+-------+---------+ | 1 | pdun | 1000 | #没有脏读 | 2 | Moker | 1000 | +----+-------+---------+ #如果终端1此时提交了 mysql> select * from user; +----+-------+---------+ | id | name | balance | +----+-------+---------+ | 1 | pdun | 1000 | | 2 | Moker | 1100 | #终端2事务并没提交，却改变了，出现了幻读 +----+-------+---------+ 2 rows in set (0.00 sec)
不存在脏读和不可重复读，存在脏读
开启终端1
#修改隔离等级
mysql> set session transaction isolation level repeatable read;
Query OK, 0 rows affected (0.00 sec)
mysql>
mysql> select @@tx_isolation;
+-----------------+
| @@tx_isolation |
+-----------------+
| REPEATABLE-READ |
+-----------------+
mysql> select *from user;
+----+-------+---------+
| id | name | balance |
+----+-------+---------+
| 1 | pdun | 1100 | #查看的一共两条数据，按理说应该两行收影响
| 2 | Moker | 1100 |
+----+-------+---------+
2 rows in set (0.00 sec)
mysql> start transaction;
mysql> update user set balance = balance+100; #这一步先不运行
---------------------------------
#开启终端2
mysql> set session transaction isolation level repeatable read;
mysql> start transaction;
Query OK, 0 rows affected (0.12 sec)
mysql> insert into user(name,balance)
-> values
-> ('Bingo',1000);
Query OK, 1 row affected (0.00 sec)
mysql> commit; #插入一个字段并提交
Query OK, 0 rows affected (0.13 sec)
-----------------------------------------------------
mysql> update user set balance = balance+100; #运行这一步
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3 Changed: 3 Warnings: 0 #3行受影响，这就是幻读