SQL query available rooms for booking - sql

Question

I have been attempting to create a query for checking available rooms between two dates but have been thus far unsuccessful. Please find the sql code for creating the tables below. I really really need your help. this is done because the system wants me to add more details
CREATE TABLE rooms (
id NUMERIC(3) NOT NULL,
type VARCHAR2(11) NOT NULL,
CONSTRAINT pk_rooms PRIMARY KEY (id),
CONSTRAINT fk_type
);
INSERT INTO rooms VALUES (101, 'Excellent');
INSERT INTO rooms VALUES (102, 'Excellent');
INSERT INTO rooms VALUES (103, 'Excellent');
INSERT INTO rooms VALUES (104, 'Excellent');
INSERT INTO rooms VALUES (105, 'Excellent');
INSERT INTO rooms VALUES (106, 'Excellent');
INSERT INTO rooms VALUES (107, 'Excellent');
INSERT INTO rooms VALUES (108, 'Excellent');
INSERT INTO rooms VALUES (109, 'Excellent');
INSERT INTO rooms VALUES (110, 'Excellent');
INSERT INTO rooms VALUES (111, 'Excellent');
INSERT INTO rooms VALUES (112, 'Excellent');
CREATE TABLE bookings_roombookings (
book_id VARCHAR(20) NOT NULL,
room_id NUMBER NOT NULL,
fromdate DATE NOT NULL,
todate DATE NOT NULL,
guest1 VARCHAR(40) NOT NULL,
guest2 VARCHAR(40),
guest3 VARCHAR(40),
CONSTRAINT cpk PRIMARY KEY (book_id, room_id),
CONSTRAINT fk_rid FOREIGN KEY (room_id) REFERENCES rooms(id));
INSERT INTO bookings_roombookings VALUES (
'DTR5000000', 320,
'01-JUN-2020', '01-SEP-2020',
'D.Trump', 'H.Clinton', 'S.Daniels');
INSERT INTO bookings_roombookings VALUES (
'DRI0000002', 102,
'11-DEC-19', '01-SEP-20',
'D.Ridley', NULL, NULL);
my query is as follows:
select r.id from rooms r
where r.id not in
( select bkr.room_id from bookings_roombookings bkr
where (fromdate > '15-JUN-20') and (todate < '18-JUN-20') );
but this will return both rows of the bkr table. I would appreciate any help.

Answer 1

You can start from the room table, and use a not exists condition with a correlated subquery to eliminate rooms that have a reservation period which overlaps the target date range:
select r.*
from rooms r
where not exists (
select 1
from bookings_roombookings bkr
where
bkr.room_id = r.id
and bkr.fromdate <= date'2020-06-15'
and bkr.todate >= date'2020-06-18'
)
Note: do not rely on implicit conversions from strings to date (as in fromdate > '15-JUN-20'), that depends on the nls settings of your database and session; instead, you can use date litterals, that are part of the ANSI SQL standard and which Oracle supports, like date'yyyy-mm-dd'.