SQL datetime compare

Try changing

15.12.2015 00:00:00 

to

2015-12-15 00:00:00

and same format for the other date also.

Even though in Europe and everywhere in the world it makes perfect sense to use the month as the second of three items in the date. In the US and in this case, apparently, SQL's date time is MM.DD.YYYY, so you're going for the 15th month and 18th month

Therefore you should use

string query "SELECT * FROM LocalHotels WHERE city='LONDON' AND start <='12.15.2015 00:00:00' AND deadline >='12.18.2015 00:00:00' ORDER BY city"

or

string query "SELECT * FROM LocalHotels WHERE city='LONDON' AND start <='2015-12-15' AND deadline >='2015-12-18' ORDER BY city"