Parking Spots Availability Design
I'd suggest to use a calendar or schedule table.

In my example I've set up a 3-day table calendar with intervals of one hour:

(Do a google search to find out some date series generator, you'll find a lot)

    Hours table
===================
2017/01/01 00:00:00
2017/01/01 01:00:00
2017/01/01 02:00:00
2017/01/01 03:00:00
2017/01/01 04:00:00
2017/01/01 05:00:00
...
...


Then simply LEFT JOIN your table with the calendar table and get NULL rows:

    Hours table               Reservations
===================        ===================
2017/01/01 00:00:00        2017/01/01 00:00:00
2017/01/01 01:00:00  LEFT  2017/01/01 01:00:00
2017/01/01 02:00:00  JOIN  NULL                 &lt;-- Free
2017/01/01 03:00:00        2017/01/01 03:00:00
2017/01/01 04:00:00        NULL                 &lt;-- Free
2017/01/01 05:00:00        NULL                 &lt;-- Free





SELECT    hdate 
FROM      calendar
LEFT JOIN reservations
ON        hdate = from_date
WHERE     parking_id is null;



dbfiddle here