![image.png](/static/img/ebd01250951aee169ba4af1d634f0b96.image.webp)

![image.png](/static/img/06161b19fc56229457543b802e7215a8.image.webp)


![image.png](/static/img/7677bdcf1286edaf52a15fdf85b02d12.image.webp)

```sql
create table Courese(
    student varchar(10) primary key,
    class varchar(15)
);

insert into Courese values ('A','math'),('B','en'),('C','math'),('D','bio'),('E','math'),('F','com'),('G','math'),('H','math'),('I','math');

select  * from Courese;

select class from Courese group by class having count(student) >=5 ;

select class from 
(select class,count(student) as num
from Courese group by class)
as tem where num >=5;

```

SQL语句练习（5）

## This a question from Quora.

***Which math isn’t used in computer science?***



***In computer science, various branches of mathematics are used extensively. However, the level of mathematical depth required can vary depending on the specific area of computer science and the type of work being done. While computer science does involve a lot of math, some areas may not require advanced mathematics like calculus or linear algebra. For example, basic arithmetic and discrete mathematics are often used in programming and algorithm design, but complex calculus or differential equations may not be necessary for many computer science applications.***

***It's important to note that the mathematical requirements can vary from one computer science specialization to another. Some areas of computer science, such as computer graphics or machine learning, may require more advanced mathematics, while others, like web development, may rely less on advanced math.***

***So, while computer science does involve a significant amount of math, the specific math you need can depend on your focus within the field.***

:::info{title="相关信息"}
在计算机科学中，广泛使用了数学的各个分支。然而，所需的数学深度水平可能取决于计算机科学的特定领域和正在进行的工作类型。虽然计算机科学涉及大量数学，但某些领域可能不需要像微积分或线性代数这样的高级数学知识。例如，在编程和算法设计中通常使用基本的算术和离散数学，但对于许多计算机科学应用而言，复杂的微积分或微分方程可能并不必要。

重要的是要注意，数学要求可能因计算机科学的不同专业而异。计算机科学的某些领域，如计算机图形学或机器学习，可能需要更高级的数学知识，而其他领域，如Web开发，可能较少依赖于高级数学。

因此，虽然计算机科学确实涉及大量数学，但你在该领域需要的具体数学可能取决于你在该领域的关注点。
:::

- Which math isn’t used：哪些数学不被使用。
- in computer science：在计算机科学中。
- various branches of mathematics：各种数学分支。
- used extensively：广泛使用。
- However：然而。
- the level of mathematical depth required：所需的数学深度水平。
- can vary depending on：可能取决于。
- the specific area of computer science：计算机科学的特定领域。
- and the type of work being done：以及正在进行的工作类型。
- While computer science does involve：虽然计算机科学涉及。
- a lot of math：大量数学。
- some areas may not require：某些领域可能不需要。
- advanced mathematics：高级数学。
- like calculus or linear algebra：比如微积分或线性代数。
- For example：例如。
- basic arithmetic and discrete mathematics：基本的算术和离散数学。
- are often used：通常被使用。
- in programming and algorithm design：在编程和算法设计中。
- but complex calculus or differential equations：但复杂的微积分或微分方程。
- may not be necessary：可能并不必要。
- for many computer science applications：对于许多计算机科学应用而言。
- It's important to note that：重要的是要注意。
- the mathematical requirements can vary：数学要求可能有所不同。
- from one computer science specialization to another：从一个计算机科学专业到另一个专业。
- Some areas of computer science：计算机科学的一些领域。
- such as computer graphics or machine learning：比如计算机图形学或机器学习。
- may require more advanced mathematics：可能需要更高级的数学。
- while others：而其他。
- like web development：比如Web开发。
- may rely less on：可能较少依赖于。
- So, while computer science does involve：因此，虽然计算机科学涉及。
- a significant amount of math：大量的数学。
- the specific math you need can depend on：你在该领域需要的具体数学可能取决于。
- your focus within the field：你在该领域的关注点。

- mathematics： 数学
- extensively： 广泛地
- depth： 深度
- required： 需要的
- vary： 变化
- depending on： 取决于
- specific： 特定的
- area： 领域
- type： 类型
- being done： 正在进行的
- involve： 涉及
- areas： 领域
- not require： 不需要
- advanced： 高级的
- calculus： 微积分
- linear algebra： 线性代数
- example： 例如
- basic： 基本的
- arithmetic： 算术
- discrete mathematics： 离散数学
- often： 通常
- used： 使用
- programming： 编程
- algorithm design： 算法设计
- complex： 复杂的
- differential equations： 微分方程
- necessary： 必要的
- computer science applications： 计算机科学应用
- important to note： 注意
- mathematical requirements： 数学要求
- specialization： 专业化
- graphics： 图形学
- machine learning： 机器学习
- rely less on： 较少依赖于
- significant amount： 大量
- specific： 具体的
- focus within the field： 在该领域的关注点

英语翻译9（Quora问题）

![image.png](/static/img/962015480a16e22079594baaa96deb0e.image.webp)



![image.png](/static/img/9c2b9997d30bda91d54e6e73156f26bf.image.webp)
```sql
create table Customers (
    id int primary key ,
    name varchar(20)
);

create table Orders (
    id int primary key ,
    customeriId int ,
    foreign key (customeriId) references Customers(id)
);

insert into Customers values (1,'Joe'),(2,'Henry'),(3,'Sam'),(4,'Max');
insert into Orders values (1,3),(2,1);

select name as Customers
from Customers as c 
where c.id not in 
(select customeriId from Orders);

select name as Customers 
from Customers left join Orders 
on Customers.id = Orders.customeriId  
where Orders.customeriId is null;
```
## 总结
- 左连接会返回左表所有记录和右表匹配记录
- 

![image.png](/static/img/4166755e571fd75d1e5ebde61a148680.image.webp)

SQL语句练习（4）

![image.png](/static/img/7feb17c5cf34ac32fd0855334f8f5249.image.webp)




![image.png](/static/img/58fa96bee3c17cf4b3b70a13b9fcf70b.image.webp)
```sql
create table Person(
    id int primary key,
    email varchar(20)
) ;



show tables;

drop table Person;

insert into Person values (1,'join@qq.com'),(2,'join@qq.com'),(3,'qwm@qq.com');
insert into Person values (5,'xwm@qq.com');
delete p1 from Person 
as p1,Person p2 where 
p1.email = p2.email 
and p1.id > p2.id;

delete from Person where
id not in (select id from
(select min(id) as id from
Person group by email) t);


select p1.email from Person as p1,
Person as p2 where p1.email=p2.email 
and p1.id!=p2.id group by p1.email;

select email from
Person group by email 
having count(email) > 1;
```
## 总结
- group by + having解决重复小妙招

SQL语句练习（3）

![image.png](/static/img/105e879534aadb6edcf64a0fb6699a4f.image.webp)

![image.png](/static/img/fbe331dd192c0371fd3e6f6517dc9857.image.webp)

也是有两种解法
```sql
create table Person(
    id int primary key,
    email varchar(20)
) ;

show tables;



drop table Person;

insert into Person values 
(1,'join@qq.com'),(2,'join@qq.com'),(3,'qwm@qq.com');

delete p1 from Person as p1,
Person p2 where p1.email = p2.email 
and p1.id > p2.id;

delete from Person where
id not in (select id from
(select min(id) as id from Person group by email) t);
```


![image.png](/static/img/60b3644e101da35cc7894f0f0c86ffdc.image.webp)

## 总结
- 不能对表同时间删除和查询
- group by 语句可以按字段分组