mysql 判断最大值,最小值,第二大,第三大 一共四个值

最大值：select max(num) from table
第二大值：select max(num) from table
where num not in(select max(num) from table)
第三大值：select max(num) from table
where num not in(select max(num) from table
where num not in(select max(num) from table))
最小值：select min(num) from table