[Leetcode]SQL语句

182. 查找重复的电子邮箱

select email from person group by email having count(email)>1

196. 删除重复的电子邮箱

delete a from 
Person a left join Person b 
on a.email = b.email 
where a.id > b.id

delete from Person where id not in(
    select a.id from 
    (select min(id) as id from Person group by email) a 
)

delete from Person where id in
(
    select id from
    (
        select a.id from Person a left join Person b on a.email = b.email where a.id > b.id
    ) as temp
)

176. 第二高的薪水

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

596. 超过5名学生的课

select class from courses group by class having count(*) >= 5

595. 大的国家

select name,population,area from world where area >= 3000000  or population >= 25000000