SQL NOT EXISTS ( BETWEEN (...) AND (...)) - sql

Question

I am creating a database that is for booking hotel rooms. I am stuck on a constraint that checks if a 'dateFrom' variable for a guest is between any 'dateFrom' and 'dateTo' variables for that specific guest. ie) a guest cannot book more than 1 room at a time.
I am getting an error: "cannot use subquery in check constraint":
CREATE TABLE tomsBooking
(
hotelNo HotelNo NOT NULL,
guestNo INT NOT NULL,
dateFrom DATE NOT NULL,
dateTo DATE NOT NULL,
roomNo RoomNumber
CONSTRAINT GuestOverlap
CHECK ( NOT EXISTS
(SELECT * FROM tomsBooking b
WHERE b.guestNo = b.guestNo
AND b.dateTo >= dateFrom
AND b.dateFrom <= dateTo
)
)
);

Answer 1

Unfortunately, Postgres does not support sub-queries for check constraints.
But this case is is exactly what exclusion constraints where created for:
CREATE TABLE tomsBooking
(
hotelNo HotelNo NOT NULL,
guestNo INT NOT NULL,
dateFrom DATE NOT NULL,
dateTo DATE NOT NULL,
roomNo RoomNumber
);
alter table tomsbooking
add constraint guestoverlap
exclude using gist (guestno with =, daterange(datefrom, dateto) with &&);
For more details and examples, see the manual: https://www.postgresql.org/docs/current/static/rangetypes.html#RANGETYPES-CONSTRAINT
In order for a GIST index to be able to use the = operator you need to install the btree_gist module using:
create extension btree_gist;
(That only needs to be done once per database)

Answer 2

Bearing in mind that I don't know Postgres SQL... it looks to me that you need the term VALUE near the BETWEEN statement so the comparison knows what value it is checking between.
As an alternative though, and based on Postgresql query between date ranges, I would structure it as this:
CREATE DOMAIN DateFrom AS DATE
CHECK (VALUE > '2016-10-16' AND NOT EXISTS (SELECT * FROM tomsBooking b
WHERE b.guestNo = g.guestNo
AND VALUE >= dateFrom
AND VALUE <= dateTo
)
);
As I say, I don't know Postgres, so you may have to tweak my suggestion.