DataBase: LeetCode

Combine Two Tables

1 # Write your MySQL query statement below

2 Select p.FirstName, p.LastName, a.City, a.State from Person p left join Address a on p.PersonId = a.PersonId

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Consecutive Numbers

1 # Write your MySQL query statement below

2 select distinct num from (

3     select num, count(rank) as cnt from (

4         select num, @cur := @cur + if(@prev = num, 0, 1) as rank, @prev := num

5         from Logs s, (select @cur := 0) r, (select @prev := null) p

6         order by id asc

7     ) t group by rank having cnt >= 3

8 ) n;

9             

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个人觉得还是用下面这个简单的好

1 # Write your MySQL query statement below

2 select distinct l1.Num from Logs l1, Logs l2, Logs l3 where l1.Id + 1 = l2.Id and l1.Id + 2 = l3.Id and l1.Num = l2.Num and l1.Num = l3.Num;

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 Customers Who Never Order

1 # Write your MySQL query statement below

2 select c.Name from Customers c left join Orders o on c.Id = o.CustomerId where o.Id is null

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1 # Write your MySQL query statement below

2 select c.Name from Customers c where c.Id not in (select CustomerId from Orders o)

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1 # Write your MySQL query statement below

2 select c.Name from Customers c where not exists (select c.Id from Orders o where o.CustomerId = c.Id)

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 in和exists的区别参考http://www.cnblogs.com/seasons1987/archive/2013/07/03/3169356.html

可以这么理解，我们偏向于：

select * from 大表 where cc in (select cc from 小表); 因为in相当于内表（这里是小表）与外表的hash连接，因此内表应选择小表

select * from 小表 where exists (select cc from 大表 where cc = 小表.cc); 因为exists是先对外表（这里是小表）做loop，再在后面exists里面的语句（内表）进行查询，因此外表要选择小表

 

Delete Duplicate Emails

1 # Write your MySQL query statement below

2 delete p1 from Person p1 inner join Person p2 where p1.email = p2.email and p1.Id > p2.Id

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1 # Write your MySQL query statement below

2 delete from p1 using Person p1 inner join Person p2 where p1.email = p2.email and p1.Id > p2.Id

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1 # Write your MySQL query statement below

2 delete from Person where Id not in (select * from (select min(Id) from Person p group by Email) t)

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注意下面这个语句会出现BUG

1 # Write your MySQL query statement below

2 delete from Person where Id not in (select min(Id) from Person p group by Email)

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错误提示为：You can't specify target table 'Person' for update in FROM clause

这个错误为在MySQL中，禁止在FROM子句中指定被更新的目标表。也就是说用Min或者max的时候要在外面再套一层select，这个错误只有在MYSQL中有，在MSSQL和Oracal中没有（好奇怪。。）

 

Department Highest Salary

1 # Write your MySQL query statement below

2 select d.name as Department, t.Salary, e.Name as Employee from Employee e inner join (select DepartmentId, max(salary) as Salary from Employee group by DepartmentId) t using (DepartmentId, Salary) inner join Department d on d.Id = t.DepartmentId

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SQL的顺序一般从from后面开始，再是select，最后是order by

下面这段代码更加容易理解

1 # Write your MySQL query statement below

2 select d.Name Department, e.Name Employee, s.Salary from (

3     select MAX(e.Salary) Salary, e.DepartmentId from Employee e, Department d where e.DepartmentId = d.Id group by e.DepartmentId

4 ) s, Employee e, Department d where s.Salary = e.Salary and e.DepartmentId = d.Id and e.DepartmentId = s.DepartmentId;

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Departmet Top Three Salaries

1 # Write your MySQL query statement below

2 select D.Name as Department, E.Name as Employee, E.Salary as Salary 

3   from Employee E, Department D

4    where (select count(distinct(Salary)) from Employee where DepartmentId = E.DepartmentId and Salary > E.Salary) in (0, 1, 2)

5          and 

6            E.DepartmentId = D.Id 

7          order by E.DepartmentId, E.Salary DESC;

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太难了。。

 

Duplicate Emails

1 # Write your MySQL query statement below

2 select distinct(p.Email) from Person p, Person q where p.Id != q.Id and p.Email = q.Email

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1 # Write your MySQL query statement below

2 select Email from Person group by Email having count(*) > 1

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Employees Earning More Than Their Managers

1 # Write your MySQL query statement below

2 select e1.Name from Employee e1, Employee e2 where e1.ManagerId = e2.Id and e1.Salary > e2.Salary

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Rank Scores

1 # Write your MySQL query statement below

2 select Score, Rank from (

3     select Score, @cur := @cur + if(@prev = Score, 0, 1) as Rank, @prev := Score from 

4         Scores s, (select @cur := 0) r, (select @prev := null) p order by Score desc) t

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Rising Temperature

1 # Write your MySQL query statement below

2 select w1.Id from Weather w1 join Weather w2 where to_days(w1.Date) = to_days(w2.Date) + 1 and w1.Temperature > w2.Temperature

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Second Highest Salary

1 # Write your MySQL query statement below

2 select max(Salary) from Employee where Salary < (select max(Salary) from Employee)

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