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;