leetcode刷题记录-数据库篇

** 1.连接考察点：**

1. inner join： 结果保留匹配的
2. left join： 结果为左边全部的
3. right join： 结果为右边全部的
4. full join： 结果为左右所有的

** 2.查询薪水第二高，空可以返回**

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

** 3.查找重复的电子邮箱**

select distinct a.Email from Person a ,Person b where a.Email = b.Email and a.id!=b.id 
#或
select Email from Person group by Email having count(*)>1