select cast(avg(abs(timestampdiff(second,ol.logtime,sl.logtime))) as SIGNED) gap
from order_log ol join select_log sl on ol.order_id=sl.order_id

abs 取绝对值，avg函数后 结果会变浮点数，所有不能直接输出，也不能用round取整，要用cast函数转化为整数类型。

可参考：https://blog.csdn.net/2301_81133727/article/details/142525132?utm_medium=distribute.pc_relevant.none-task-blog-2~default~baidujs_utm_term~default-0-142525132-blog-134595984.235^v43^pc_blog_bottom_relevance_base8&spm=1001.2101.3001.4242.1&utm_relevant_index=3