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;