目录

1.每位经理的下属员工数量

 2.员工的直属部门

 3.判断三角形

 4.上级经理已离职的公司员工

 5.换座位

6.电影评分

7.修复表中的名字

 8.患某种疾病的患者

9.删除重复的电子邮箱 

1.每位经理的下属员工数量

# Write your MySQL query statement below
#e1是经理，e2是员工
select e1.employee_id,e1.name,count(e2.employee_id) as reports_count,round(avg(e2.age)) as average_age
from Employees e1
join Employees e2
on e1.employee_id=e2.reports_to
group by e1.employee_id,e1.name
order by e1.employee_id;

 2.员工的直属部门

# Write your MySQL query statement below
select employee_id,if(count(department_id)=1,department_id,
            max(case primary_flag when 'Y' then department_id end)) as department_id
from Employee
group by employee_id

 3.判断三角形

# Write your MySQL query statement below
select x,y,z,
      case
          when x+y>z and x+z>y and y+z>x then 'Yes'
          else 'No'
      end as triangle
from Triangle;

 4.上级经理已离职的公司员工

# Write your MySQL query statement below
#子查询就是查询套查询
select employee_id
from Employees 
where salary<30000
and manager_id not in (select employee_id from Employees)
order by employee_id

 5.换座位

# Write your MySQL query statement below
select s1.id as id,
      #逻辑：如果没有下一位，且当前是奇数，不改动；奇数取下一位的值，偶数取上一位的值
      if(s2.id is null && s1.id%2=1,s1.student,if(s1.id%2=1,s2.student,s3.student)) as student
from Seat s1
left join Seat s2 on s2.id=s1.id+1
left join Seat s3 on s3.id=s1.id-1
order by id

6.电影评分

 

# Write your MySQL query statement below
#查找用户
(select u.name as results
from Users u
left join MovieRating mr 
on u.user_id=mr.user_id
group by u.user_id
order by count(*) desc,name asc
limit 1
)
#查找电影
union all
(
select title as results
from Movies m
left join MovieRating mr
on m.movie_id=mr.movie_id and year(mr.created_at)=2020 and month(mr.created_at)=2
group by mr.movie_id
order by avg(mr.rating) desc,title asc
limit 1
)

7.修复表中的名字

# Write your MySQL query statement below
#使用substring分割字符，使用upper,lower大小写，使用concat连接
select user_id,concat(upper(substring(name,1,1)),lower(substring(name,2))) name
from Users
order by user_id

 8.患某种疾病的患者

# Write your MySQL query statement below
select *
from Patients
where conditions regexp '^DIAB1|\\sDIAB1'

 

9.删除重复的电子邮箱 

 

# Write your MySQL query statement below
#delete删除
delete p1
from Person p1,Person p2
where p1.email=p2.email and p1.id>p2.id