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事务:事务由单独单元的一个或多个SQL语句组成,在这个单元中,每个MySQL语句是相互依赖的。而整个单独单元作为一个不可分割的整体,如果单元中某条SQL语句一旦执行失败或产生错误,整个单元将会回滚。所有受到影响的数据将返回到事物开始以前的状态;如果单元中的所有SQL语句均执行成功,则事物被顺利执行。
隐式事务:事务没有明显的开启和结束的标记
比如insert、update、delete语句显式事务:事务具有明显的开启和结束的标记
前提:必须先设置自动提交功能为禁用 set autocommint=0步骤:
步骤1:开启事务
set autocommit=0; start transaction;(可选的) 步骤2:编写事务中的sql语句(select insert update delete) 步骤3:结束事务 commit;提交事务 rollback;回滚事务#开启事务SET autocommit = 0;START TRANSACTION;#编写一组事务的语句UPDATE account SET balance = 500 WHERE username='张无忌';UPDATE account SET balance = 1500 WHERE username='赵敏';#结束事务COMMIT;#ROLLBACK;
同时运行的多个事务, 当这些事务访问数据库中相同的数据时, 如果没有采取必要的隔离机制, 就会导致各种并发问题:
- | 脏读 | 不可重复读 | 幻读 |
---|---|---|---|
read uncommitted | ✔ | ✔ | ✔ |
read committed | × | ✔ | ✔ |
repeatable read | × | × | ✔ |
serializable | × | × | × |
Oracle 支持的 2 种事务隔离级别:READ COMMITED(默认), SERIALIZABLE。
Mysql 支持 4 种事务隔离级别.默认为REPEATABLE READ 一些命令:#查看当前隔离级别SELECT @@tx_isolation;#设置当前 mySQL 连接的隔离级别:set transaction isolation level read committed;#设置数据库系统的全局的隔离级别:set global transaction isolation level read committed;
SET autocommit=0;START TRANSACTION;DELETE FROM account WHERE id=1;SAVEPOINT a;DELETE FROM account WHERE id=2;ROLLBACK TO a;
注意:
delete可以回滚,truncate删除表后无法回滚含义:虚拟表,和普通表一样使用,是通过表动态生成的数据
MySQL从5.0.1版本开始提供视图功能。一种虚拟存在的表,行和列的数据来自定义视图的查询中使用的表,并且是在使用视图时动态生成的,只保存了sql逻辑,不保存查询结果 应用场景:多个地方用到同样的查询结果、该查询结果使用的sql语句较复杂SELECT stuname, majorname FROM stuinfo sINNER JOIN major m ON s.majorID=m.idWHERE s.stuName LIKE '张%';CREATE VIEW v1 AS SELECT stuname, majorname FROM stuinfo sINNER JOIN major m ON s.majorID=m.idWHERE s.stuName LIKE '张%';SELECT * FROM v1 WHERE stuname LIKE '张%';
语法:create view 视图名
as 查询语句;#1.查询邮箱中包含a字符的员工名、部门名和工种信息#①创建CREATE VIEW myv1ASSELECT last_name,department_name,job_titleFROM employees eJOIN departments d ON e.department_id = d.department_idJOIN jobs j ON j.job_id = e.job_id;#②使用SELECT * FROM myv1 WHERE last_name LIKE '%a%';#2.查询各部门的平均工资级别#①创建视图查看每个部门的平均工资CREATE VIEW myv2ASSELECT AVG(salary) ag, department_idFROM employees GROUP BY department_id;#②使用SELECT myv2.ag, g.grade_levelFROM myv2 JOIN job_grades gON myv2.ag BETWEEN g.lowest_sal AND g.highest_sal#3.查询平均工资最低的部门信息SELECT * FROM myv2 ORDER BY ag LIMIT 1;#4.查询平均工资最低的部门名和工资CREATE VIEW myv3 AS SELECT * FROM myv2 ORDER BY ag LIMIT 1;SELECT d.*, m.agFROM myv3 m JOIN departments dON m.department_id = d.department_id;
方式一:
create or replace view 视图名 as 查询语句;SELECT * FROM myv3;CREATE OR REPLACE VIEW myv3 AS SELECT AVG(salary),job_id FROM employeesGROUP BY job_id;
方式二:
alter view 视图名 as 查询语句;ALTER VIEW myv3ASSELECT * FROM employees;
删除
语法:drop view 视图名,视图名; 查看 DESC SHOW CREATE VIEW 视图名;如果不进行操作,在增删改视图的情况下就可以更改表中数据
SELECT * FROM employees;SELECT * FROM empv1;#1.插入INSERT INTO empv1 VALUES('张飞','zf@163.com');#2.修改UPDATE empv1 SET last_name = '张无忌' WHERE last_name='张飞';#3.删除DELETE FROM empv1 WHERE last_name = '张无忌';
具有以下特点的视图不允许更新
#①包含以下关键字的sql语句:分组函数、distinct、group by、having、union或者union allCREATE OR REPLACE VIEW empv2AS SELECT MAX(salary), department_id FROM employeesGROUP BY department_id;SELECT * FROM empv2;#更新UPDATE empv2 SET m=9000 WHERE department_id=10;#②常量视图CREATE OR REPLACE VIEW empv2ASSELECT 'jhon' NAME;SELECT * FROM empv2;#更新UPDATE empv2 SET NAME='lucy';#③SELECT中包含子查询CREATE OR REPLACE VIEW empv2AS SELECT (SELECT MAX(salary) FROM employees) 最高工资;#更新SELECT * FROM empv2;UPDATE empv2 SET 最高工资=100000;#④joinCREATE OR REPLACE VIEW empv2AS SELECT last_name, department_nameFROM employees e JOIN departments dON e.department_id = d.department_id;#更新SELECT * FROM empv2;UPDATE empv2 SET last_name='张飞' WHERE last_name='Whalen';INSERT INTO empv2 VALUES('陈真','xxxxx');#⑤from一个不能更新的视图#⑥where子句的子查询引用了from子句中的表CREATE VIEW myv4ASSELECT last_name,email,salary FROM employeesWHERE employee_id IN( SELECT manager_id FROM employees WHERE manager_id IS NOT NULL);#更新SELECT * FROM myv4;UPDATE myv6 SET salary=10000 WHERE last_name = 'K_ing';
- | 创建语法的关键字 | 是否占用事积的物理空间 |
---|---|---|
视图 | create view | 保存了sql逻辑 |
表 | create table | 保存了数据 |
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