SELECT
    CAST(AVG(ABS(timestampdiff(second,ol.logtime,sl.logtime))) AS SIGNED) AS gap
FROM
    order_log ol 
JOIN
    select_log sl ON ol.order_id=sl.order_id

CAST(... AS SIGNED) 是 MySQL 中的类型转换函数，意思是：把某个表达式转换成有符号整数类型（带正负号的整数）