数据库的锁、事务

时间 2019-12-10 标签数据库事务

链接管理器：
接收请求
建立线程
认证用户
进来安全链接mysql

处理并发，多版本访问控制MVCC
每一个用户在不一样的时间快照上修改，修改完毕以后，数据库再合并数据修改状况。web

锁：

按照锁的施行方式分类：读锁、写锁

读锁：共享锁，能够容许其余人读，可是不容许他人写
写锁：独占锁，不容许其余人的读、写操做sql

LOCK TABLES tb_name [read|write] UNLOCK TABLES

按照锁的影响范围（锁粒度）分类

按照锁的影响范围（锁粒度）分为表锁、页锁（block）、行锁。MySQL服务器仅支持表级锁，行锁须要存储引擎完成。数据库

锁表实例

session 1：将表stu_test锁定为读锁安全

mysql> lock tables stu_test read;
Query OK, 0 rows affected (0.00 sec)

session 2：尝试读取和插入操做ruby

mysql> select * from stu_test; +----+------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 | +----+------+-----+--------+-----+------+
| 1 | lina | 3 | F | 33 | NULL | +----+------+-----+--------+-----+------+
1 row in set (0.01 sec)

mysql> insert into stu_test (name,cid,gender,age,cid2) values ('jim',2,'M',23,3);

能够进行读操做，可是没法进行写操做。在插入数据时会阻塞，等待表解锁。bash

session 1：释放锁，session 2中的插入语句执行。服务器

mysql> unlock tables;
Query OK, 0 rows affected (0.01 sec)

mysql> select * from stu_test; +----+------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 | +----+------+-----+--------+-----+------+
|  1 | lina |   3 | F      |  33 | NULL |
| 2 | jim | 2 | M | 23 | 3 | +----+------+-----+--------+-----+------+
2 rows in set (0.00 sec)

session 2：显示插入数据成功。session

mysql> insert into stu_test (name,cid,gender,age,cid2) values ('jim',2,'M',23,3);
Query OK, 1 row affected (3 min 49.03 sec)

事务

RDBMS：ACID（原子性、一致性、隔离性、持久性）并发

原子性：一个操做要么都完成，要么所有失败，好比转帐，A转B，要么A转出成功，B接收成功，要么都失败。
一致性：事务的完成和完成以后的结果要一致。好比转帐结束后，二者的总和是同样的。
隔离性：第一个事务执行不能影响到第二个事务的执行。
持久性：一个事务执行完毕以后，就算服务器宕机，服务器开机以后，事务仍是存在的。内存中事务执行完毕以后会存储到物理设备上。

事务日志分为重作日志和撤销日志

重作日志(redo log）：事务的操做在真正写到数据库中时，先写到日志当中。若是数据库崩溃，会根据重作日志进行从新写入。

撤销日志(undo log）:每一次操做，在操做以前将原有的状态保存。当操做失败，能够还原日志

事务的执行过程：事务的操做都是在内存中完成而后写到事务日志中，过段时间，提交日志，修改物理设备中的数据。[执行语句写入事务日志] + [根据日志修改物理实例存储]

DELETE语句的操做，先在日志中标记删除，当事务中的语句都执行完毕，会将全部的操做写在日志中。最后日志提交以后会将数据同步到数据文件中。

日志组：事务的日志文件保存多个，若是只有一个日志，事务未执行完毕可是日志文件写满须要等待提交事务日志，严重影响执行效率。若是是两个日志文件，当第一个日志文件写完，能够写第二个日志文件，提交第一个日志。

事务的隔离性：

若是多个事务同时运行，第一个事务的执行对第二个事务的影响有隔离级别的区分

隔离级别：
READ-UNCOMMITTED: 读未提交，第一我的有删除操做，未提交事务，第二我的立马能看到。隔离级别最低，事务直接的影响最大。
READ-COMMITED: 读提交，第一个执行操做提交事务才能看到。会致使第二我的在事务执行时，能看到数据，第二次执行操做时发现没法找到数据。
REPATABLE-READ ：可重读，第一个事务的执行、提交不会影响第二个事务的执行，可是在第二个事务的提交时可能会致使和执行的时候不一样。（MySQL）
SERIALIZABLE：可串行。第一个事务彻底执行完毕物理存储文件中的数据修改完毕执行第二个事务。

mysql> show global variables like '%iso%';

+---------------+-----------------+
| Variable_name | Value | +---------------+-----------------+
| tx_isolation | REPEATABLE-READ | +---------------+-----------------+
1 row in set (0.00 sec)

事务的隔离性测试

读未提交

session 1：设置隔离级别为读未提交

mysql> set tx_isolation='READ-UNCOMMITTED'; Query OK, 0 rows affected (0.00 sec) mysql> select @@tx_isolation; +------------------+
| @@tx_isolation | +------------------+
| READ-UNCOMMITTED | +------------------+
1 row in set (0.00 sec)

session 2：设置隔离级别为读未提交

mysql> set tx_isolation='READ-UNCOMMITTED'; Query OK, 0 rows affected (0.00 sec) mysql> select @@tx_isolation; +------------------+
| @@tx_isolation | +------------------+
| READ-UNCOMMITTED | +------------------+
1 row in set (0.00 sec)

Session 1和session 2 都启动事务：

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

session 1：修改数据

mysql> update  student set age = 22 where id = 14;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

session 2 ：查询修改后的数据

mysql> select * from student;
+----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+--------+-----+--------+-----+------+
|  2 | tom |   2 | M      |  19 | NULL |
|  3 | jack |   3 | M      |  20 |    4 |
|  4 | lucy |   2 | F      |  25 |    4 |
|  5 | neccy |   3 | F      |  30 | NULL |
|  6 | mary |   4 | F      |  21 |    5 |
|  7 | kaka |   5 | M      |  21 |    3 |
|  8 | suke |   3 | M      |  20 |    5 |
|  9 | suke |   4 | M      |  21 |    3 |
| 10 | beita |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita |   3 | NULL   |  22 |    1 |
+----+--------+-----+--------+-----+------+
13 rows in set (0.00 sec)

session 1：回滚数据

mysql> rollback;
Query OK, 0 rows affected (0.00 sec)

session 2：查看数据

mysql> select * from student;
+----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+--------+-----+--------+-----+------+
|  2 | tom |   2 | M      |  19 | NULL |
|  3 | jack |   3 | M      |  20 |    4 |
|  4 | lucy |   2 | F      |  25 |    4 |
|  5 | neccy |   3 | F      |  30 | NULL |
|  6 | mary |   4 | F      |  21 |    5 |
|  7 | kaka |   5 | M      |  21 |    3 |
|  8 | suke |   3 | M      |  20 |    5 |
|  9 | suke |   4 | M      |  21 |    3 |
| 10 | beita |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita |   3 | NULL   |  32 |    1 |
+----+--------+-----+--------+-----+------+
13 rows in set (0.00 sec)

总结：session1 中的数据修改，尚未提交，session2中就能够看到。会致使session2中的数据先后查询的结果不一样。

读提交

session 1 和session 2：设置隔离级别为读未提交

mysql> set tx_isolation='READ-COMMITTED'; Query OK, 0 rows affected (0.00 sec) mysql> select @@tx_isolation; +----------------+
| @@tx_isolation | +----------------+
| READ-COMMITTED | +----------------+
1 row in set (0.00 sec)

session 1 和session 2：启动事务

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

session 1 修改数据库中的数据

mysql> update tb1 set age=33 where id=2;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from tb1; +------+------+------+
| id | name | age | +------+------+------+
|    1 | tom  |   30 |
| 2 | jim | 33 | +------+------+------+
2 rows in set (0.02 sec)

session 2 ：查看数据，没有改变

mysql> select * from tb1; +------+------+------+
| id | name | age | +------+------+------+
|    1 | tom  |   30 |
| 2 | jim | 35 | +------+------+------+
2 rows in set (0.02 sec)

session 1 ：提交事务

mysql> commit;
Query OK, 0 rows affected (0.03 sec)

session 2 ：查看数据

mysql> select * from tb1; +------+------+------+
| id | name | age | +------+------+------+
|    1 | tom  |   30 |
| 2 | jim | 33 | +------+------+------+
2 rows in set (0.02 sec)

总结：session 1中的事务没有提交，session2中的数据没有变化，若是session1中的事务提交完毕，session2 没有执行完毕，会致使先后执行的结果不一样。

事务的可重读

session1 和 session 2 ：同时设置事务的隔离等级

mysql> set tx_isolation='REPEATABLE-READ'; Query OK, 0 rows affected, 1 warning (0.02 sec) mysql> 
mysql> select @@tx_isolation; +-----------------+
| @@tx_isolation | +-----------------+
| REPEATABLE-READ | +-----------------+
1 row in set, 1 warning (0.03 sec)

session1 和 session 2 ：启动事务

mysql> start transaction;
Query OK, 0 rows affected (0.02 sec)

mysql> select * from tb1; +------+------+------+
| id | name | age | +------+------+------+
|    1 | tom  |   30 |
| 2 | jim | 31 | +------+------+------+
2 rows in set (0.02 sec)

session1 ：修改数据并提交

mysql> update tb1 set age=55 where id = 2;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from tb1; +------+------+------+
| id | name | age | +------+------+------+
|    1 | tom  |   30 |
| 2 | jim | 55 | +------+------+------+
2 rows in set (0.03 sec)

mysql> commit;
Query OK, 0 rows affected (0.03 sec)

mysql> select * from tb1; +------+------+------+
| id | name | age | +------+------+------+
|    1 | tom  |   30 |
| 2 | jim | 55 | +------+------+------+
2 rows in set (0.02 sec)

session2 ：查看数据是否修改

mysql> select * from tb1; +------+------+------+
| id | name | age | +------+------+------+
|    1 | tom  |   30 |
| 2 | jim | 31 | +------+------+------+
2 rows in set (0.02 sec)

session2 ：查看提交先后数据的修改。

mysql> commit;
Query OK, 0 rows affected (0.02 sec)

mysql> select * from tb1; +------+------+------+
| id | name | age | +------+------+------+
|    1 | tom  |   30 |
| 2 | jim | 55 | +------+------+------+
2 rows in set (0.03 sec)

总结：session1 中的数据修改、事务提交以后，session2中的数据是不一样的，可是session2中的事务提交先后不一样。

可串行

session1 和 session2 ：设置事务的隔离级别

mysql> set tx_isolation = 'SERIALIZABLE'; Query OK, 0 rows affected, 1 warning (0.02 sec) mysql> select @@tx_isolation; +----------------+
| @@tx_isolation | +----------------+
| SERIALIZABLE | +----------------+
1 row in set, 1 warning (0.02 sec)

session1 和 session2 ：启动事务

mysql> start transaction;
Query OK, 0 rows affected (0.02 sec)

session1 ：修改数据

mysql> select * from tb1; +------+------+------+
| id | name | age | +------+------+------+
|    1 | tom  |   30 |
| 2 | jim | 55 | +------+------+------+
2 rows in set (0.02 sec)

mysql> update tb1 set age=88 where id = 2;
Query OK, 1 row affected (0.04 sec)
Rows matched: 1  Changed: 1  Warnings: 0

session2：查询数据，发现数据卡顿甚至延迟较高致使中断。

mysql> select * from tb1;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

session1：提交事务

mysql> commit
    -> ; Query OK, 0 rows affected (0.04 sec)

session2：查询数据

mysql> select * from tb1; +------+------+------+
| id | name | age | +------+------+------+
|    1 | tom  |   30 |
| 2 | jim | 88 | +------+------+------+
2 rows in set (0.02 sec)

总结：session1,没有执行完毕，session2 是没法执行的。只有session1中的事务提交以后session2中的数据才能显示。

易错点

若是在同一个会话中设置这四种隔离模式会致使，现象出错。
缘由：使用的事务不是对应的事务的隔离级别
解决方案：
1.每一次测试从新链接
2.在作下一次测试时，使用rollback ，多回滚几回。

事务的特性：

automicity：原子性，事务所引发的数据库操做，要么完成，要么都不执行
consistency：一致性，
isolation：隔离性，
事务调度：事务之间的影响最小
MVCC: 多版本并发控制
durability：持久性，一旦事务完成都要保证事
1.事务提交以前就已经写出数据至持久性存储
2.结合书屋日志完成
事务日志：顺序IO
数据文件：随机IO

事务的状态

事务状态	解释
活动的：Active	事务正在执行中
部分提交	最后一条语句执行，最后一条语句执行后，数据写到磁盘上
失败	事务正常提交了，可是未能完成提交
停止的	事务没有提交
提交的	事务提交，并成功提交

事务并发执行：
1.提升吞吐量和资源的利用率
2.减小等待时间

并发控制依赖的技术手段:
锁
时间锁
多版本和快照隔离

饿锁：锁饥饿，事务没法调度锁
死锁：

事务调度：
可恢复调度
无级联调度

事务的启动

可以启动事务的操做，通常是一堆SQL语句，或是是ODBC中启动事务的指令。

start tansaction：启动事务的指令
SQL语句，（事务的操做指令）
commit：提交，（事务完毕）
rollback ：事务回滚

事务的自动提交

若是没有明确的启动事务，
autocommit：能实现自动提交，每个操做都直接提交

建议：明确使用事务，而且关闭自动提交。

经过变量查看是设置自动提交的

mysql> select @@autocommit; +--------------+
| @@autocommit | +--------------+
| 1 | +--------------+
1 row in set (0.00 sec)

关闭自动提交

mysql> set autocommit = 0;
Query OK, 0 rows affected (0.00 sec)

mysql> select @@autocommit; +--------------+
| @@autocommit | +--------------+
| 0 | +--------------+
1 row in set (0.00 sec)

回滚测试

mysql> delete from student where id = 3;
Query OK, 1 row affected (0.00 sec)

mysql> select * from student;
+----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+--------+-----+--------+-----+------+
|  2 | tom |   2 | M      |  19 | NULL |
|  4 | lucy |   2 | F      |  25 |    4 |
|  5 | neccy |   3 | F      |  30 | NULL |
|  6 | mary |   4 | F      |  21 |    5 |
|  7 | kaka |   5 | M      |  21 |    3 |
|  8 | suke |   3 | M      |  20 |    5 |
|  9 | suke |   4 | M      |  21 |    3 |
| 10 | beita |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita |   3 | NULL   |  32 |    1 |
+----+--------+-----+--------+-----+------+
12 rows in set (0.00 sec)

mysql> rollback;
Query OK, 0 rows affected (0.02 sec)

mysql> select * from student;
+----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+--------+-----+--------+-----+------+
|  2 | tom |   2 | M      |  19 | NULL |
|  3 | jack |   3 | M      |  20 |    4 |
|  4 | lucy |   2 | F      |  25 |    4 |
|  5 | neccy |   3 | F      |  30 | NULL |
|  6 | mary |   4 | F      |  21 |    5 |
|  7 | kaka |   5 | M      |  21 |    3 |
|  8 | suke |   3 | M      |  20 |    5 |
|  9 | suke |   4 | M      |  21 |    3 |
| 10 | beita |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita |   3 | NULL   |  32 |    1 |
+----+--------+-----+--------+-----+------+
13 rows in set (0.00 sec)

保存点

mysql> help savepoint
Name: 'SAVEPOINT'
Description:
Syntax:
SAVEPOINT identifier
ROLLBACK [WORK] TO [SAVEPOINT] identifier
RELEASE SAVEPOINT identifier

mysql> start transaction;    # 启动事务
Query OK, 0 rows affected (0.00 sec)

mysql> select * from student;
+----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+--------+-----+--------+-----+------+
|  2 | tom |   2 | M      |  19 | NULL |
|  3 | jack |   3 | M      |  20 |    4 |
|  4 | lucy |   2 | F      |  25 |    4 |
|  5 | neccy |   3 | F      |  30 | NULL |
|  6 | mary |   4 | F      |  21 |    5 |
|  7 | kaka |   5 | M      |  21 |    3 |
|  8 | suke |   3 | M      |  20 |    5 |
|  9 | suke |   4 | M      |  21 |    3 |
| 10 | beita |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita |   3 | NULL   |  32 |    1 |
+----+--------+-----+--------+-----+------+
13 rows in set (0.00 sec)

mysql> savepoint stu0;    # 设置事务的保存点
Query OK, 0 rows affected (0.00 sec)

mysql> delete from student where id = 2;
Query OK, 1 row affected (0.01 sec)

mysql> select * from student;
+----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+--------+-----+--------+-----+------+
|  3 | jack |   3 | M      |  20 |    4 |
|  4 | lucy |   2 | F      |  25 |    4 |
|  5 | neccy |   3 | F      |  30 | NULL |
|  6 | mary |   4 | F      |  21 |    5 |
|  7 | kaka |   5 | M      |  21 |    3 |
|  8 | suke |   3 | M      |  20 |    5 |
|  9 | suke |   4 | M      |  21 |    3 |
| 10 | beita |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita |   3 | NULL   |  32 |    1 |
+----+--------+-----+--------+-----+------+
12 rows in set (0.00 sec)

mysql> savepoint stu2;
Query OK, 0 rows affected (0.00 sec)

mysql> delete from student where id = 3;
Query OK, 1 row affected (0.00 sec)

mysql> savepoint stu3;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from student;
+----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+--------+-----+--------+-----+------+
|  4 | lucy |   2 | F      |  25 |    4 |
|  5 | neccy |   3 | F      |  30 | NULL |
|  6 | mary |   4 | F      |  21 |    5 |
|  7 | kaka |   5 | M      |  21 |    3 |
|  8 | suke |   3 | M      |  20 |    5 |
|  9 | suke |   4 | M      |  21 |    3 |
| 10 | beita |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita |   3 | NULL   |  32 |    1 |
+----+--------+-----+--------+-----+------+
11 rows in set (0.00 sec)

mysql> rollback to stu0;     #恢复保存点
Query OK, 0 rows affected (0.00 sec)

mysql> select * from student;
+----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 |
+----+--------+-----+--------+-----+------+
|  2 | tom |   2 | M      |  19 | NULL |
|  3 | jack |   3 | M      |  20 |    4 |
|  4 | lucy |   2 | F      |  25 |    4 |
|  5 | neccy |   3 | F      |  30 | NULL |
|  6 | mary |   4 | F      |  21 |    5 |
|  7 | kaka |   5 | M      |  21 |    3 |
|  8 | suke |   3 | M      |  20 |    5 |
|  9 | suke |   4 | M      |  21 |    3 |
| 10 | beita |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita |   3 | NULL   |  32 |    1 |
+----+--------+-----+--------+-----+------+
13 rows in set (0.00 sec)

mysql> commit
    -> ;
Query OK, 0 rows affected (0.01 sec)

事务回滚测试

事务提交以前能够回滚。

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> use students;
Database changed
mysql> select * from students;
ERROR 1146 (42S02): Table 'students.students' doesn't exist
mysql> select * from student; +----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 | +----+--------+-----+--------+-----+------+
|  1 | jim    |   1 | M      |  21 |    1 |
|  2 | tom    |   2 | M      |  19 | NULL |
|  3 | jack   |   3 | M      |  20 |    4 |
|  4 | lucy   |   2 | F      |  25 |    4 |
|  5 | neccy  |   3 | F      |  30 | NULL |
|  6 | mary   |   4 | F      |  21 |    5 |
|  7 | kaka   |   5 | M      |  21 |    3 |
|  8 | suke   |   3 | M      |  20 |    5 |
|  9 | suke   |   4 | M      |  21 |    3 |
| 10 | beita  |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita | 3 | NULL | 32 | 1 | +----+--------+-----+--------+-----+------+
14 rows in set (0.00 sec)

mysql> delete from student where id = 1;
Query OK, 1 row affected (0.01 sec)

mysql> select * from student; +----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 | +----+--------+-----+--------+-----+------+
|  2 | tom    |   2 | M      |  19 | NULL |
|  3 | jack   |   3 | M      |  20 |    4 |
|  4 | lucy   |   2 | F      |  25 |    4 |
|  5 | neccy  |   3 | F      |  30 | NULL |
|  6 | mary   |   4 | F      |  21 |    5 |
|  7 | kaka   |   5 | M      |  21 |    3 |
|  8 | suke   |   3 | M      |  20 |    5 |
|  9 | suke   |   4 | M      |  21 |    3 |
| 10 | beita  |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita | 3 | NULL | 32 | 1 | +----+--------+-----+--------+-----+------+
13 rows in set (0.00 sec)

mysql> rollback
 -> ;
Query OK, 0 rows affected (0.01 sec)

mysql> select * from student; +----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 | +----+--------+-----+--------+-----+------+
|  1 | jim    |   1 | M      |  21 |    1 |
|  2 | tom    |   2 | M      |  19 | NULL |
|  3 | jack   |   3 | M      |  20 |    4 |
|  4 | lucy   |   2 | F      |  25 |    4 |
|  5 | neccy  |   3 | F      |  30 | NULL |
|  6 | mary   |   4 | F      |  21 |    5 |
|  7 | kaka   |   5 | M      |  21 |    3 |
|  8 | suke   |   3 | M      |  20 |    5 |
|  9 | suke   |   4 | M      |  21 |    3 |
| 10 | beita  |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita | 3 | NULL | 32 | 1 | +----+--------+-----+--------+-----+------+
14 rows in set (0.00 sec)


mysql> select * from student; +----+--------+-----+--------+-----+------+
| id | name | cid | gender | age | cid2 | +----+--------+-----+--------+-----+------+
|  2 | tom    |   2 | M      |  19 | NULL |
|  3 | jack   |   3 | M      |  20 |    4 |
|  4 | lucy   |   2 | F      |  25 |    4 |
|  5 | neccy  |   3 | F      |  30 | NULL |
|  6 | mary   |   4 | F      |  21 |    5 |
|  7 | kaka   |   5 | M      |  21 |    3 |
|  8 | suke   |   3 | M      |  20 |    5 |
|  9 | suke   |   4 | M      |  21 |    3 |
| 10 | beita  |   3 | M      |  24 |    1 |
| 11 | wukong |   5 | M      |  25 |    2 |
| 12 | wujing |   1 | M      |  28 | NULL |
| 13 | wuneng |   4 | F      |  30 | NULL |
| 14 | tita | 3 | NULL | 32 | 1 | +----+--------+-----+--------+-----+------+
13 rows in set (0.00 sec)

事务提交

事务提交以后没法回滚。

mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)

mysql> delete from student where id = 1;
Query OK, 1 row affected (0.00 sec)

mysql> commit;
Query OK, 0 rows affected (0.01 sec)