I can't write a query to save my life or have data to test against. So, if you run the following, I hope you have a backup or expect errors... otherwise, here's a bump for you.
Code:
SELECT
t1.checkid,
t1.checkurl,
t1.seconds,
t2.checktime as checktime
FROM
Check_urls as t1
LEFT JOIN Check_log as t2 on t1.checkid = t2.checkid
WHERE (t2.checkid is null OR (t1.checktime < ( {dynamic_timestamp}-t1.seconds ) ) )