My accepted solution, 813ms
select  IF(count(*) >= N, Min(rank.Salary), NULL) salary\n    from (select distinct salary\n          from Employee     \n          order by salary desc\n      
    select  IF(count(*) >= N, Min(rank.Salary), NULL) salary\n    from (select distinct salary\n          from Employee     \n          order by salary desc\n          limit N\n     ) rank
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