SQL Date overlap avoidance inside of the CREATE statement using CHECK constraint - sql

Question

I am really stuck on this because I dont understand how can I check for possible overlaps in such cases. So this is the condition of the task:
Imagine you are the manager of a hotel that has a table in its database with the following definition:
CREATE TABLE Hotel
(
Room SMALLINT NOT NULL,
Arrival DATE NOT NULL,
Departure DATE NOT NULL,
Guest CHAR(30),
PRIMARY KEY (room number, arrival)
CHECK (departure >= arrival)
);
So you can't leave this hotel before you've arrived. Now modify this definition so that no reservation can be entered in the table such as the Arrival/Departure date conflicts with an already existing reservation date. As overlaps count (examples):
already existing reservation 3.1.-6.1. with newly booked 1.1.‐5.1. or 4.1.-10.1.
or another existing 2.1.-6.1. with newly booked 1.1.-10.1. or 3.1.-5.1.
It also states, that it's OK to use selections from the Table inside of the CREATE statement of this same table, and use it in a CHECK constraint.

Answer 1

Before reading check the tutorial about triggers and how they work https://www.postgresqltutorial.com/postgresql-triggers/creating-first-trigger-postgresql/
CREATE TABLE if not exists Hotel
(
Room SMALLINT NOT NULL,
Arrival DATE NOT NULL,
Departure DATE NOT NULL,
Guest CHAR(30),
PRIMARY KEY (room, arrival),
CHECK (departure >= arrival)
);
CREATE OR REPLACE FUNCTION register_new_insert()
RETURNS TRIGGER
AS
$$
DECLARE
r hotel%rowtype;
BEGIN
FOR r IN
SELECT * FROM hotel WHERE Room = New.Room;
LOOP
IF r.arrival < new.arrival and new.arrival < r.departure
THEN RAISE EXCEPTION 'Arrival/Departure date conflicts with an already existing reservation date';
END IF;
IF r.arrival < new.departure and new.departure < r.departure
THEN RAISE EXCEPTION 'Arrival/Departure date conflicts with an already existing reservation date';
END IF;
END LOOP;
RETURN NEW;
END;
$$ LANGUAGE PLPGSQL;
CREATE TRIGGER register_new
BEFORE insert
ON Hotel
FOR EACH ROW
EXECUTE PROCEDURE register_new_insert();

Answer 2

You can create a daterange with existing arrival, depature columns and also with the new values, then apply the range overlaps operator (&&). (See demo);
create or replace function prevent_register_overlap()
returns trigger
language plpgsql
as $$
begin
if exists ( select null
from hotel h
where daterange(new. arrival, new.departure,'[]') &&
daterange(h.arrival, h.departure,'[]')
and new.room = h.room
)
then raise exception 'Arrival/Departure overlaps existing reservation.';
end if;
return new;
end;
$$;
create trigger register_new
before insert or update of arrival, departure
on hotel
for each row
execute procedure prevent_register_overlap();
As constructed these are closed ranges,both dates are included, see 8.17.6. Constructing Ranges for creating other types.