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#select email from (select email, count(*) as sum from Person group by email) tmp where tmp.sum >= 2;#ÒÔÓʼþ×ֶνøÐзÖ×飬ȻºóÓÃhaving½øÐйýÂËselect Emailfrom Persongr Read More

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