User
------------------
ID
Name


Room
------------------
ID
Subject



Participate
-------------------
ID
room_id
user_id

user                        room                    participate
-----------------           ----------------        ---------------------
ID     Name                 ID     subject          ID  room_id  user_id
-----------------           ----------------        ---------------------
1      john                 1      room1            1     1       1
2      sara                 2      room2            2     1       2
3      david                3      room3            3     3       3
                                                    4     3       2
                                                    5     3       1
                                                    6     2       1