年少有为

#select email from (select email, count(*) as sum from Person group by email) tmp where tmp.sum >= 2;#以邮件字段进行分组，然后用having进行过滤select Emailfrom Persongr Read More

select IFNULL((select distinct salary from Employee order by salary desc limit 1, 1), NULL) as SecondHighestSalary; Read More