I have a table with `timestamp` field and `expiry` field.
the `expiry` contain number of minutes (eg 15, 30, 60... )

I want to select only the rows that the diff between NOW() and the `timestamp`
field is larger the the value in `expiry`.

something like that:

```
select * FROM signals 
WHERE min(time_to_sec(timediff(now(), stime)))/60 > expiry

```