Control Language things: to handle unexpected disruptions

thing

A set of SQL statements or an execution unit, either all executed or not executed the whole

Fails, an error, the entire unit will be rolled back. (Undo all operations)

All data affecting the return to the state before the start of something.

If the unit all SQL statements executed successfully, then things are smoothly executed

Have the properties of things: ACID (acid) property

1.A Atomicity: things are representative of an indivisible unit of work (minimum unit), performing either all, or does not perform full
2.C consistency: a thing will perform data, from one consistent state, is switched to, another consistent state. (Accurate, complete and reliable)
3.I isolated: a thing of execution, interference from other things (isolation level control)
4.D persistence: Once the data is submitted to a permanent change things, and the next operation failure does not affect it

Deleted is deleted, and no way to withdraw, only to re-insert

View support MySQL storage engine

SHOW ENGINES;

InnoDB	The current server default storage engine	Support thing
MEMORY	Many also use	Does not support things
MyISAM	Previously used version 5.5	Does not support things

Created things

Implicit things (automatic thing)

Things did not clear the opening and mark the end of the (automatically turned on automatically end)
such as insert, update, delete statement

Display things

Mark has obvious things open and end

The two statements and implicit thing as a
prerequisite: you must disable automatic submission
disable automatic submission: set autocommit = 0;
only being valid for the current transaction, not permanently closed

案例：转账
张三丰  1000
郭襄    1000
UPDATE 表 SET 张三丰余额=500 WHERE NAME='张三丰'
意外中断
UPDATE 表 SET 郭襄余额=1000 WHERE NAME='郭襄'3
钱错了

但是update本身就是一个事物，所以必须禁用自动提交

显示自动提交的状态

SHOW VARIABLES LIKE 'autocommit';

autocommit自动提交

value on 表示开启

1.事物控制的步骤：

#步骤1：开启事物
set autocommit=0;#禁用自动提交，写了这个就默认开启事物
start transaction；#可选的
#步骤2：编写事物中的SQL语句
（只支持 insert update delete，增删改查才有用。select可以使用但是没什么意义）ddl语言没有事物之说
语句1；
语句2；
...
#步骤3：结束事物
commit;提交事物，没有意外，语句可以全部执行
rollback;回滚事物，有意外回滚，语句不能全部执行

MySQL没有办法演示事物出现异常回滚，没有异常提交，jdbc可以。

单独无法达到更具情况，必须联合应用程序

#演示事物的使用步骤

#1.开启事物
SET autocommit=0;
START TRANSACTION;

#2.编写一组事物的语句
UPDATE account SET balance =500 WHERE username='张无忌';
UPDATE account SET balance =1500 WHERE username='赵敏';

#3.结束事物
COMMIT;#提交事物，更改
------------------------------------------------------------------------------------
#1.开启事物
SET autocommit=0;
START TRANSACTION;

#2.编写一组事物的语句
UPDATE account SET balance =1000 WHERE username='张无忌';
UPDATE account SET balance =1000 WHERE username='赵敏';

#3.结束事物
ROLLBACK;#回滚事物，撤销

在结束事物之前数据存储在内存，并没有提交到磁盘文件。

执行到结束任务，才决定：提交到磁盘文件、还是撤销

2.delete和truncate在事物中使用时的区别

delete 可以回滚撤销
truncate 不支持回滚撤销

#delete演示
SET autocommit=0;
START TRANSACTION;

DELETE FROM account;
ROLLBACK;

SELECT * FROM account;

#truncate演示
SET autocommit=0;
START TRANSACTION;
TRUNCATE TABLE account;
ROLLBACK;

3.演示 savepoint 的使用

savepoint 节点名（自定义）；设置保存点
搭配
ROLLBACK TO 节点名;#回滚到保存点，撤销保存点后的操作

#查询表中的数据
SELECT * FROM account;

#1.开启事务
SET autocommit=0;
START TRANSACTION; 
#2.删除
DELETE FROM account WHERE id=1;
savepoint a;#设置保存点
DELETE FROM account WHERE id=3;
ROLLBACK TO a;#回滚到保存点
只删除了：1,3的操作被撤销了

4.并发事物

多个事物，同时操作数据库中相同数据。如果没有采取必要的隔离机制，就会导致各种并发问题

类似多线程的线程安全，五个人同时上一个厕所，所以要启用加锁的方式，实现线程同步，解决

脏读：读到了其他事物更新但没有提交的数据

不可重复读：一个事物多次读取，到的结果不一样。

幻读：读到了其他事物插入但没提交的数据
第一个事物更新之前是三条数据，其他事物还没有提交或添加。这时候第二个事物提交了数据，第一个事物一更新影响了四行数据。

可以通过设计数据库的隔离级别：来解决并发问题。

从上往下级别逐次增高，性能从上往下逐次降低

	脏读	不可重复读	幻读
读未提交 read uncommitted	√	√	√
读已提交 read committed	×	√	√
可重复读 repeatable read	×	×	√
串行化 serializable	×	×	×

MySQL默认第三个隔离级别：repeatable read
oracle默认第二个隔离级别：read committed，还支持第四个：serializable

查看隔离级别

设置隔离级别只针对当前连接有效，
登录一个是一个连接，查询不是连接都是一个连接下的

select @@tx_isolation

设置当前隔离级别

set session transaction isolation level 隔离级别;

设置全局隔离级别

set global transaction isolation level 隔离级别;

TCL- things isolation level