select cast(avg(case when timestampdiff(second,o.logtime,s.logtime)>=0 then timestampdiff(second,o.logtime,s.logtime)
else -timestampdiff(second,o.logtime,s.logtime) end )as signed) as gap 
from order_log o
join select_log s
on o.order_id = s.order_id

avg会导致算出的值是浮点数，因此需要用cast（字段 as signed）取整