SELECT 
    cast(avg(abs(# 根据题目意思，先abs后avg
        timestampdiff(second,
        order_log.logtime,
        select_log.logtime)
    )) AS signed) AS gap #需要使用cast函数将timestamp映射成整数值
FROM order_log
LEFT JOIN select_log
ON order_log.order_id=select_log.order_id;