正则， $末尾 , ^  非,   +一个或多个
select regexp_substr(o.order_id,"[^_]+$") as product_id, count(*)
from order_log o 
group by product_id

select substring_index(order_id,'_',-1) as product_id, count(1) as cnt
from order_log
group by substring_index(order_id,'_',-1)
order by 2 desc
limit 1

正则， $末尾 , ^ 非, +一个或多个