数据库—子查询，视图，事务

一、子查询

子查询：将一条查询sql的结果作为另一条查询sql的条件

增

#insert into 表 select 子查询

查

# select fron 表 where() in (select 子查询)[表不能与delete表相同]
'''查每个学校身高最高的那个人的全部信息'''

1）#子查询的sql
select school,max(height) from student group by school;
#先查询出每个学校的最大身高
+--------+-------------+
| school | max(height) |
+--------+-------------+
| 北大   | 183.1       |
| 复旦   | 183.3       |
| 清华   | 185.9       |
+--------+-------------+

2）#查
select * from student where (school,height) in (select school,max(height) from student group by school);
+----+------+--------+--------+--------+--------+--------+
| id | name | gender | height | area   | port   | school |
+----+------+--------+--------+--------+--------+--------+
|  2 | bbb  | 男     | 183.1  | 上海   | 青浦   | 北大   |
|  3 | ccc  | 男     | 183.3  | 北京   | 朝阳   | 复旦   |
|  4 | ddd  | 男     | 185.9  | 广东   | 广州   | 清华   |
+----+------+--------+--------+--------+--------+--------+

# 将子查询转换为一张表

1)# 创建一个存子查询数据的一张表
create table school_height(
school_name varchar(64),
max_height varchar(64));

2)#将子查询的数据增加到school_height表中
insert into school_height select school,max(height) from student group by school;
+-------------+------------+
| school_name | max_height |
+-------------+------------+
| 北大        | 183.1      |
| 复旦        | 183.3      |
| 清华        | 185.9      |
+-------------+------------+

3）#需求(同样也会实现上面的需求)
select name,school_name,height
from student join school_height
on student.school = school_height.school_name
and student.height = school_height.max_height;
+------+-------------+--------+
| name | school_name | height |
+------+-------------+--------+
| bbb  | 北大        | 183.1  |
| ccc  | 复旦        | 183.3  |
| ddd  | 清华        | 185.9  |
+------+-------------+--------+

改

# update 表 set 字段=值 where 字段 in (select 子查询)[表不能与delete表相同]

'''每个部门最大身高加一'''
update school_height set max_height=max_height+1;
+-------------+------------+
| school_name | max_height |
+-------------+------------+
| 北大        | 184.1      |
| 复旦        | 184.3      |
| 清华        | 186.9      |
+-------------+------------+

'''给school_height表额外增加一个新学校'''
insert into school_height values ('南开', 188);
+-------------+------------+
| school_name | max_height |
+-------------+------------+
| 北大        | 184.1      |
| 复旦        | 184.3      |
| 清华        | 186.9      |
| 南开        | 188        |
+-------------+------------+

'''更改'''
update school_height set max_height=max_height+1 where school_name in (select distinct school from student);
+-------------+------------+
| school_name | max_height |
+-------------+------------+
| 北大        | 185.1      |
| 复旦        | 185.3      |
| 清华        | 187.9      |
| 南开        | 188        |
+-------------+------------+

#You can't specify target table 'school_height' for update in FROM clause
#报错，update更新的表 与 子查询select的表 相同
update school_height set max_height=max_height+1 
where school_name in 
(select distinct school_name from school_height);

删

#delete from 表 where() in (select 子查询)[表不能与delete表相同]

delete from school_height where school_name in (select distinct school from student);
+-------------+------------+
| school_name | max_height |
+-------------+------------+
| 南开        | 188        |
+-------------+------------+


#错误: delete删除的表 与 子查询select的表 相同
delete from school_height where school_name in (select distinct school_name from school_height);

二、区间修饰条件：all与any

all:要满足所有的条件

where salary < all(3, 6, 9) => salary必须小于所有情况

any:满足其中一个条件就行

where salary < any(3, 6, 9) => salary只要小于一种情况

eg：
select * from student where height < all(select height from student where id<4);
#先找到id<4的学生的身高，分别为 173.1，183.1，183.3，之后再拿身高作为参照物，找出满足所有满足身高条件（身高小于173.1）的同学
+----+------+--------+--------+--------+--------------+--------+
| id | name | gender | height | area   | port         | school |
+----+------+--------+--------+--------+--------------+--------+
|  5 | eee  | 女     | 168    | 山东   | 烟台         | 北大   |
|  6 | fff  | 女     | 165    | 新疆   | 乌鲁木齐     | 北大   |
|  8 | hhh  | 女     | 166.1  | 广州   | 广东         | 复旦   |
| 13 | ppp  | 女     | 155    | 江苏   | 连云港       | 清华   |
+----+------+--------+--------+--------+--------------+--------+

三、视图：view

视图的增删改操作可以直接映射给真是 表（本质就是对真是表进行操作）

• 视图是存在内存中的临时表
• 视图的创建依赖select语句
• 视图支持对数据的增删改查
• 视图不允许对视图表的字段做修改
• 视图不仅支持创建，也支持更新与删除

'''创建视图'''
'''create view 视图名[(别名们)] as select 语句'''

create view 视图名[(别名们)] as select 语句;
eg:
create view v1 as select 
school,max(height) 
from student group by school;
+--------+-------------+
| school | max(height) |
+--------+-------------+
| 北大   | 183.1       |
| 复旦   | 183.3       |
| 清华   | 185.9       |
+--------+-------------+

'''创建或替换视图'''
''' create or replace 视图名[(别名们)] as select 语句;'''
create or replace view v1(school_name, max_height) as select school, max(height) from student group by school;
+-------------+------------+
| school_name | max_height |
+-------------+------------+
| 北大        | 183.1      |
| 复旦        | 183.3      |
| 清华        | 185.9      |
+-------------+------------+

''' alter 视图名[(别名们)] as select 语句;'''
alter view v1(name, height) as select school, max(height) from student group by school;
+--------+--------+
| name   | height |
+--------+--------+
| 北大   | 183.1  |
| 复旦   | 183.3  |
| 清华   | 185.9  |
+--------+--------+

'''删除视图'''
drop view 视图名
eg：drop view v1;

'''视图可以作为正常表完成连表查询'''
eg：create or replace view v2 as select name,gender,height from student;
select * from v2;
+------+--------+--------+
| name | gender | height |
+------+--------+--------+
| aaa  | 女     | 173.1  |
| bbb  | 男     | 183.1  |
| ccc  | 男     | 183.3  |
| ddd  | 男     | 185.9  |
| eee  | 女     | 168    |
| fff  | 女     | 165    |
.........

视图的增删改

视图可以完成增删改，增删改本质是对真实的表进行操作

#先创建一个视图
create or replace view v2 as select id,name,gender,height from student;

#改
update v2 set height=height+1 where id=1; #两个表都改变了
select * from v2;
+----+------+--------+--------+
| id | name | gender | height |
+----+------+--------+--------+
|  1 | aaa  | 女     | 174.1  |

select * from student;
+----+------+--------+--------+-----------+--------------+--------+
| id | name | gender | height | area      | port         | school |
+----+------+--------+--------+-----------+--------------+--------+
|  1 | aaa  | 女     | 174.1  | 甘肃      | 张掖         | 清华   |


#删
deleate from v2 where id = 1;

#增
create or replace view v3 as select * from student;
insert into v2 values(18,'wwb','男','183','上海','青浦','北大');

#操作视图会影响真实表，反之也会影响
update sutdent set height = height+1  whereid= 1；

四、事务

通常一些业务需要多条sql参与，参与的sql会形成一个 执行整体，该整体我们就称之为事务（事务就是包含多条执行的sql语句）

4.1事务的四大特性

• 原子性：所有的执行都要一次性成功，不能有任何一个执行失败，要么同时成功，要么同时失败
• 一致性：事务前后的数据完整性应该保持一致
• 隔离性：支持并发，执行可以同时进行，数据不会乱
• 持久性：一旦事务被提交，他对数据的改变就是永久性的，接下来如果数据库有故障，也不会对提交的数据有影响

create table bank(
id int primary key auto_increment,
name varchar(16),
money decimal(65,2));

insert into bank(name,money) values('yjy',10),('wwb',20);

#假设出现以下执行情况
update bank set money = money-1 where name = 'yjy';
update bank set money = money+1 where name = 'wwb';

'''没有事务支持下，yjy的钱就丢了'''
#这种执行是错误的，因为ypp不存在，yjy的银行账户钱转丢了
update bank set money = money-1 where name = 'yjy';
update bank set money = money+1 where name = 'ypp'; 

'''将2条sql看做事务处理'''
#开启事务
begin;
update bank set money = money-1 where name = 'yjy';
update bank set money = money+1 where name = 'ypp'; 
#确认无误，提交事务
commit;
#确认有误，回滚事务
rollback;