求最大的出现一次的数字 表嵌套
# 只出现一次的最大数
num
8
8
2
2
5
5
2
7
结果是7
num
8
8
2
2
5
5
1
1
结果是null
select max(num) as num
from mynumbers
where num in (
select num
from mynumbers
group by num
having count(num)<=1)
;