13事务
Posted direwolf22
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1、什么叫做事务?
mysql中,事务其实是一个最小的不可分割的工作单元。事务能够保证一个业务的完整性。
eg.银行转账:
a要向b转出100元:a-->-100:
update user set money=money-100 where name=‘a‘;
b-->+100:
update user set money=money+100 where name=‘b‘;
实际的程序中,如果上述只有一条语句成功了,而另外一条没有执行成功的话,就会出现数据前后不一致。
因此,多条sql语句,可能会有同时执行成功的要求,要么就同时失败。
2、mysql中如何控制事务?(commit、rollback)
mysql默认是开启事务的(自动提交)。
mysql> select @@autocommit; +--------------+ | @@autocommit | +--------------+ | 1 | +--------------+
默认开启事务的作用是什么?
当我们执行一个sql语句的时候,效果会立即体现出来,且不能回滚。
mysql> create database bank; mysql> use bank; mysql> create table user( -> id int primary key, -> name varchar(20), -> money int -> ); mysql> INSERT INTO user VALUES(1,‘a‘,1000);
事务回滚:撤销sql语句执行效果
mysql> rollback; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 1000 | +----+------+-------+
尽管使用rollback,user的数据还在,没有撤回。
解决:设置mysql自动提交为false:
mysql> set autocommit=0; mysql> select @@autocommit; +--------------+ | @@autocommit | +--------------+ | 0 | +--------------+
结果:
mysql> INSERT INTO user VALUES(2,‘b‘,1000); mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 1000 | | 2 | b | 1000 | +----+------+-------+ mysql> rollback; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 1000 | +----+------+-------+
再一次插入数据:
手动提交数据(commit),这时候再撤销是行不通的。(持久性产生效果,rollback无用了)
mysql> INSERT INTO user VALUES(2,‘b‘,1000); mysql> commit; mysql> rollback; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 1000 | | 2 | b | 1000 | +----+------+-------+
自动提交
@@autocommit=1
手动提交
commit;
事务回滚
rollback;
转账:
mysql> update user set money=money-100 where name=‘a‘; mysql> update user set money=money+100 where name=‘b‘;
mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 900 | | 2 | b | 1100 | +----+------+-------+ mysql> rollback; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 1000 | | 2 | b | 1000 | +----+------+-------+
事务为我们提供了一个可以反悔的机会。
3、手动开启事务:begin 和 start transaction
还原自动提交设置:
mysql> set autocommit=1; mysql> select @@autocommit; +--------------+ | @@autocommit | +--------------+ | 1 | +--------------+
begin; 或者 start transaction; 都可以帮我们手动开启一个事务。
mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 1000 | | 2 | b | 1000 | +----+------+-------+
转账后:
mysql> update user set money=money-100 where name=‘a‘; mysql> update user set money=money+100 where name=‘b‘; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 900 | | 2 | b | 1100 | +----+------+-------+
此时,事务回滚没有被撤销,rollback无用(因为已设置成自动提交):
mysql> rollback; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 900 | | 2 | b | 1100 | +----+------+-------+
再次转账:
mysql> begin; mysql> update user set money=money-100 where name=‘a‘; mysql> update user set money=money+100 where name=‘b‘; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 800 | | 2 | b | 1200 | +----+------+-------+
rollback之后:
mysql> rollback; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 900 | | 2 | b | 1100 | +----+------+-------+
使用begin(start transaction)手动开启事务,事务可以回滚。
mysql> start transaction; mysql> update user set money=money-100 where name=‘a‘; mysql> update user set money=money+100 where name=‘b‘; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 800 | | 2 | b | 1200 | +----+------+-------+
mysql> rollback; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 900 | | 2 | b | 1100 | +----+------+-------+
事务开启之后,一旦commit提交,就不可以回滚(当前这个事务在提交的时候就结束了)。
mysql> commit; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 900 | | 2 | b | 1100 | +----+------+-------+ mysql> rollback; mysql> select * from user; +----+------+-------+ | id | name | money | +----+------+-------+ | 1 | a | 900 | | 2 | b | 1100 | +----+------+-------+
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