数据库练习

select (select distinct Salary  from Employee order by Salary desc limit 1,1) as SecondHighestSalary

select ifnull((select distinct Salary  from Employee order by Salary desc limit 1,1),null) as SecondHighestSalary

create function getNthHighestSalary(N int) returns int
begin 
	declare P int default N-1;
	if (P<0)  then return null;
	else return (
		select ifnull(
		(
			select distinct Salary from Employee order by Salary desc limit P,1
		),null) as getNthHighestSalary
	);
	end if;
end 

select a.Score , count(distinct b.Score) rank
from Scores a ,Scores b 
where b.Score >= a.Score 
group by a.id order by a.Score desc;

select a.Score ,(
    select count(distinct b.Score) from Scores b 
    where b.Score >= a.Score  
　　)rank from Scores a order by a.Score desc 

select a.Name Employee
from Employee a,Employee b
where a.ManagerId = b.Id and a.Salary > b.Salary
group by a.Id 

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

select b.Name Department, a.Name Employee, a.Salary Salary 
from Employee a join Department b
on a.DepartmentId = b.Id 
where 3>(
    select count(distinct c.Salary) from Employee c 
    where c.Salary > a.Salary and a.DepartmentId = c.DepartmentId
) # 关键

select tt.Department, tt.Employee, tt.Salary from (
    select te.Department,te.Employee,
    case 
        when @p = ID then @r := @r + 1
        when @p := ID then @r:=1
    end as rank1,
    @rk := (case when @sa = te.Salary then @rk else @r end) rank ,
    @sa := te.Salary Salary
    from (select @p:=0,@r:=0,@rk:=0,@sa:=0) ss,(
        select a.DepartmentId ID, b.Name Department ,a.Name Employee ,a.Salary Salary 
        from Employee a left join Department b
        on a.DepartmentId = b.Id 
        order by a.DepartmentId , a.Salary desc
    ) te
) tt where tt.rank <=3;

delete a from Person a ,Person b
where a.Email = b.Email and a.Id > b.Id   

select t.Request_at Day ,    
round(sum(case when t.Status <> ‘completed‘ then 1 else 0 end)/count(t.Request_at),2) as `Cancellation Rate` from 
(
select * from Trips where Client_Id <> (
    select Users_Id from Users where Banned = ‘Yes‘ and Role = ‘client‘  
    
)
)t 
where t.Request_at between "2013-10-01" and "2013-10-03" 
group by t.Request_at  

select a.Request_at Day,
    round(sum(case when a.Status <> ‘completed‘ then 1 else 0 end)/count(a.Request_at),2) as `Cancellation Rate` from 
    Trips a left join Users b
    on a.Client_Id = b.Users_Id 
    where  b.Banned ="No" and a.Request_at between "2013-10-01"  and "2013-10-03"
    group by a.Request_at

select a.Request_at Day, 
    round(sum( if(a.Status = ‘completed‘,0,1) ) / count(a.Status),2) as `Cancellation Rate`
    from Trips a join Users b on
    a.Client_Id = b.Users_Id and b.Banned = ‘No‘
    join Users c on
    a.Driver_Id = c.Users_Id and c.Banned = ‘No‘
    where a.Request_at between "2013-10-01"  and "2013-10-03"
    group by a.Request_at

select a.request_at as Day, 
	round( 
		sum( 
				if (a.Status = ‘completed‘,0,1)
			) 
			/ 
			count(a.Status),
			2
		) as `Cancellation Rate`
from trips a left join 
(
	select users_id from Users 
	where banned = ‘Yes‘
) b on (a.Client_Id  = b.users_id)
left join (
	select users_id from Users 
	where banned = ‘Yes‘
) c on (a.Driver_Id = c.users_id)
where b.users_id is null and c.users_id is null
and a.Request_at  between ‘2013-10-01‘ and ‘2013-10-03‘
group by a.Request_at;

select
	(case 
		when id % 2 = 0 then id - 1
		when id = (select max(id) from seat) then id 
		else id +1 end) id  , student 
	from seat order by id ;

select 
	if (mod(id,2)=0,id-1,if(id =(select max(id) from seat),id,id+1)) id ,student
	from seat 
	order by id ;

( select s1.id ,s2.student
from seat s1, seat s2
where s1.id = s2.id -1 and s1.id MOD 2 =1
 )
union 
( select s1.id, s2.student 
from seat s1, seat s2 
 where s1.id = s2.id + 1 and s1.id MOD 2 = 0
)
union 
(
select s1.id, s1.student
from seat s1, seat s2
where s1.id mod 2 = 1 and s1.id = (select max(id) from seat ) 
)
order by id