Create combination sql table - sql

Question

I'm trying to create a sql table in data base in VS that has room and userid column, but the sql will only accept your input if the userid exists in users table and room exists in rooms tables
Allows:
Users table:
Userid
1
2
3
RoomUsers table:
Room ----- User
1 1
2. 1
1. 2
1. 3
2. 3
Won't allow:
Users table:
Userid
1
2
RoomUsers table:
Room ----- User
1 4
Normal foreign key wont work because it only allows one of each index and not multiple, how can I allow what I need to occur,to happen?

Answer 1

(This would be a mess in comments)
Probably we are having an XY problem here. The thing you describe is simply solved with a foreign key. ie:
CREATE TABLE users (id INT IDENTITY NOT NULL PRIMARY KEY, ad VARCHAR(100));
CREATE TABLE rooms (id INT IDENTITY NOT NULL PRIMARY KEY, ad VARCHAR(100));
CREATE TABLE room_user
(
RoomId INT NOT NULL
, UserId INT NOT NULL
, CONSTRAINT PK_roomuser
PRIMARY KEY(RoomId, UserId)
, CONSTRAINT fk_room
FOREIGN KEY(RoomId)
REFERENCES dbo.rooms(id)
, CONSTRAINT fk_user
FOREIGN KEY(UserId)
REFERENCES dbo.users(id)
);
INSERT INTO dbo.users(ad)
OUTPUT
Inserted.id, Inserted.ad
VALUES('RayBoy')
, ('John')
, ('Frank');
INSERT INTO dbo.rooms(ad)
OUTPUT
Inserted.id, Inserted.ad
VALUES('Room1')
, ('Room2')
, ('Room3');
INSERT INTO dbo.room_user(RoomId, UserId)VALUES(1, 1), (1, 2), (2, 3);
-- won't allow
INSERT INTO dbo.room_user(RoomId, UserId)VALUES(999, 888);