SQL查询练习一(From LeetCode)

 请选用MySQL进行测试.

1.找出后一天比前一天温度高的日期(E)

思路:将该表进行自关联,然后选择出日期靠后的一天,最后选择温度高的日期

 select Weather.Id
 from Weather join Weather w
 on datediff(Weather.Date,w.Date) = 1
 and Weather.Temperature > w.Temperature

2.找出人口在3百万或者领土面积在25百万的国家(E)

思路:使用where进行条件过筛选,or进行或判断

 select name,population,area
 from World
 where area > 3000000 or population > 25000000

 1 select name,population,area
 2 from World
 3 where area > 3000000 or population > 2500000

3.判断是否是三角形(E)

思路,使用case when进行搭配,使用三角形定义进行判断x+y>z,x+z>y,y+z>x

 select x,y,z,
 case
     when x+y>z and y+z>x and x+z>y then 'Yes'
     else 'No'
 end as triangle
 from triangle 

4.找出薪水第二高的员工

思路:先找出最多的薪水的员工,在把他的薪水小于最大的工资即可

 select Max(Salary) as SecondHighestSalary
 from Employee
 where Salary < (select Max(Salary) from Employee)

5.找出每个学科都有多少名学生(M)

思路:将两张表进行左连接,一department表作为主表,然后按照dept_name进行分组,最后按照人数进行降序排列

 select d.dept_name,count(student_id) as student_number
 from department d left join Student s
 on d.dept_id = s.dept_id
 group by d.dept_name
 order by student_number desc,d.dept_name

6.找出每个部门薪水最高的员工(M)

思路:将两张表进行连接,内层查询根据department表的name进行分组,每组的最大值,既是每个部门的薪水最大值,然后传递给外层的部门id和薪水即可

 select d.Name as Department,e.Name as Employee,e.Salary as Salary
 from Department d join Employee e
 on e.DepartmentId = d.Id
 where (e.DepartmentId,e.Salary) in
 (
     select DepartmentId,max(Salary)
     from Employee
     group by DepartmentId
 )

7.找出至少有5名下属的领导(M)

思路:使用内层查询查找出有5名下属的ManagerId然后,将外层查询的员工Id=ManagerId就是查询的结果

 select e1.Name
 from Employee e1
 join
 (
     select ManagerId from Employee
     group by ManagerId
     having count(*) >= 5
 ) as e2
 on e1.Id = e2.ManagerId

8.找出得票最多的候选人(M)

思路:先在内层查询中找出最受欢迎的候选人,然后将中间表的候选人Id既是赢家的候选人id,两者相等即可

 select c.Name
 from Candidate c
 join
 (
     select CandidateId  from Vote
     group by CandidateId
     order by count(*) desc
     limit 1
 ) as winner
 on c.id = winner.CandidateId;

9.根据Score计算等级(M)

思路:将两张表进行自连接,根据Id进行分组,最后根据Rank进行排序

 select s.Score,count(distinct t.Score) as Rank
 from Scores s join Scores t
 on s.Score <= t.Score
 group by s.Id
 order by Rank 

10.找出二叉树的节点分布(M)

思路:使用case when的结构进行循环判断输出

 select id,
 case
     when tree.id = (select atree.id from tree atree where atree.p_id is NULL)
         then 'Root'
     when tree.id in (select atree.p_id from tree atree)
         then 'Inner'
     else
         'Leaf'
 end as Type
 from tree
 order by id

11.找出每个部门薪水排前三名的员工(H)

思路:先进行表连接,将内层查询的结果和外部的表的Salary相比较,选择前面3个

 select d.Name as Department,e.Name as Employee,e.Salary
 from Employee e join Department d
 on e.DepartmentId = d.Id
 where 3 >
 (
     select count(distinct e2.Salary)
     from Employee e2
     where e2.Salary > e.Salary
     and e.DepartmentId = e2.DepartmentId
 )

12.找出2013-10-01到2013-10-03之间的网约车的取消率(H)

思路:计算取消率,使用case when语法,找出Trips中Status变量以canceled_开头的比例

 select t.Request_at as Day,
 round(sum(case when t.Status like 'cancelled_%' then 1 else 0 end)/count(*),2) as "Cancellation Rate"
 from Trips t join Users u
 on t.Client_Id = u.Users_Id
 where u.Banned = 'No'
 and t.Request_at between '2013-10-01' and '2013-10-03'
 group by t.Request_at

13.找出每个部门员工薪水的中位数(H)

思路:将此表进行自关联,计算工资的中位数,使用case when计算中间表的中位数

 select e.Id,e.Company,e.Salary
 from Employee e join Employee aliens
 on e.Company = aliens.Company
 group by e.Company,e.Salary
 having sum(case when e.Salary = aliens.Salary then 1 else 0 end) >=
 abs(sum(sign(e.Salary-aliens.Salary)))
 order by e.Id