SQL - Find free slots for booking system

I got a simple mysql table for already booked rooms. Here the sql structure:

CREATE TABLE Rooms (
   `ID` int, `Description` varchar(50)
);

INSERT INTO Rooms values
(123, 'Room in Frankfurt'),
(234, 'Room in Wiesbaden'),
(245, 'Room in Darmstadt');

CREATE TABLE Bookings (
   `ID` int, `StartTime` datetime, `EndTime` datetime, `post_ID` int
);

INSERT INTO Bookings
    (`ID`, `StartTime`, `EndTime`, `post_ID`)
VALUES
    (1, '2018-01-05', '2018-04-05', 123),
    (2, '2018-08-01', '2018-10-01', 123),
    (3, '2019-02-01', '2019-06-01', 123),
    (4, '2018-02-01', '2018-06-01', 234),
    (5, '2018-08-01', '2018-09-01', 294),
    (6, '2018-09-01', '2018-11-30', 234),
    (7, '2018-11-01', '2018-12-30', 294)
;

In this table we can see all bookings for all rooms we have. My problem is to find a SQL Query to find free slots for a room. The user can give these parameters:

a date for the earliest check-in (ex: 2018-10-01)
a date for the latest check-in (ex: 2018-10-15)
a maximum period in months (example: 3 months)

So the user needs a room for 3 months, starting from 01-15. October 2018.

How can i do this? I really don't get it.

Thanks!

Solution

I assume that you have some kind of Rooms table

http://sqlfiddle.com/#!9/fe977/1

SELECT r.*
FROM rooms r
LEFT JOIN Bookings b
ON r.id = b.post_id
   AND (
     (b.StartTime <= '2018-10-01'
   AND b.EndTime >= '2018-10-01')
     OR
     (b.StartTime >= '2018-10-01'
   AND b.StartTime <= '2018-10-15')
   )     
WHERE b.id IS NULL

UPDATE I am still not sure if I've got your goal. But here is another approach. If you want start date to be flexible, I would recommend to set calendar table for all dates in the year. This will allow to avoid unnecessary calculations when you run query.

Here is a fiddle: http://sqlfiddle.com/#!9/29926/3

SELECT r.*,
c.date
FROM rooms r
LEFT JOIN calendar c
ON c.date BETWEEN '2018-10-01' AND '2018-10-15'
LEFT JOIN Bookings b
ON r.id = b.post_id
   AND (
     (b.StartTime <= c.date
   AND b.EndTime >= c.date)
     OR
     (b.StartTime >= c.date
   AND b.StartTime <= (c.date + INTERVAL 3 MONTH))
   )     
WHERE b.id IS NULL

UPDATE 2 Combining with @Strawberry answer I guess we can modify query to:

http://sqlfiddle.com/#!9/29926/5

SELECT r.*,
c.date
FROM rooms r
LEFT JOIN calendar c
ON c.date BETWEEN '2018-10-01' AND '2018-10-15'
LEFT JOIN Bookings b
ON r.id = b.post_id
   AND b.StartTime <= (c.date + INTERVAL 3 MONTH)
   AND b.EndTime >= c.date     
WHERE b.id IS NULL