Second lowest is a bit of a pain. But, you can do it in MySQL with the substring_index()
/group_concat()
trick:
Create table LOW_HIGH_ALL as
select ID, FIeld1, FIeld2, max(High), min(Low),
substring_index(substring_index(group_concat(distinct low order by low asc), ',', 2), ',', -1) as min_2
from LOW_HIGH
group by Field1, Field2;
This concatenates the "low" values together and then extracts the second from the resulting string.