事务

1 定义：

如果一个包含多个步骤的业务操作，被事务管理，要么同时成功，要么同时失败。
1.1 举例

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转账操作站在数据库的角度看有以上3步。在没有被事务管理时，如果第2步出现异常，可能会出现张三的账户少了500，李四的账号没收到这500。
被事务管理，开启事务后，如果在第2步出现了异常，则进行回滚，张三账户金额不变，提示你转账失败。如果顺利完成转账的3步，最后要提交事务，整个事务就完成了。

2 事务的操作

2.1 开启事务 ：start transaction
2.2 回滚:rollback，回滚到开启事务的时候
2.3 提交事务:commit,结束事务
2.4 例子：
2.4.1 预置条件

--创建一个account表
create table account (id int primary key auto_increment,name varchar(20),balance decimal(10,2));
--插入数据
mysql> insert account (name,balance) values ('zhangsan',1000.00),('lisi',1000.00);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0
mysql> select * from account;
+----+----------+---------+
| id | name     | balance |
+----+----------+---------+
|  1 | zhangsan | 1000.00 |
|  2 | lisi     | 1000.00 |
+----+----------+---------+
2 rows in set (0.00 sec)

2.4.1 没有设置事务，在从张三转500给李四的过程中出错。

---省略查看账户余额这一步
--从zhangsan的账户中减去500；
mysql> update account set balance = balance - 500.00 where name = 'zhangsan';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0
---把lisi的名字写错成lishi，lisi的账户未增加500；
mysql> update account set balance = balance + 500.00 where name = 'lishi';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0  Changed: 0  Warnings: 0
--张三的500块不见了
mysql> select * from account;
+----+----------+---------+
| id | name     | balance |
+----+----------+---------+
|  1 | zhangsan |  500.00 |
|  2 | lisi     | 1000.00 |
+----+----------+---------+
2 rows in set (0.00 sec)

mysql> 

2.4.2 设置事务，过程中发现出错，rollback会见状态还原到开启事务时。

mysql> select * from account;
+----+----------+---------+
| id | name     | balance |
+----+----------+---------+
|  1 | zhangsan | 1000.00 |
|  2 | lisi     | 1000.00 |
+----+----------+---------+
2 rows in set (0.00 sec)
---开启事务
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> update account set balance = balance - 500.00 where name = 'zhangsan';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0
---发现操作出错，lisi的名字打错成lishi
mysql> update account set balance = balance + 500.00 where name = 'lishi';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0  Changed: 0  Warnings: 0
----回滚，回到开启事务时的状态
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from account;
+----+----------+---------+
| id | name     | balance |
+----+----------+---------+
|  1 | zhangsan | 1000.00 |
|  2 | lisi     | 1000.00 |
+----+----------+---------+
2 rows in set (0.01 sec)

2.4.3 设置事务，过程中没有出错，操作完成后，进行提交，结束该事务。

mysql> update account set balance = balance - 500.00 where name = 'zhangsan';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> update account set balance = balance + 500.00 where name = 'lisi';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from account;
+----+----------+---------+
| id | name     | balance |
+----+----------+---------+
|  1 | zhangsan |  500.00 |
|  2 | lisi     | 1500.00 |
+----+----------+---------+
2 rows in set (0.00 sec)
--提交，结束事务
mysql> commit;
Query OK, 0 rows affected (0.01 sec)