CREATE TABLE Courses(
   code CHAR(6) PRIMARY KEY,
   cname TEXT NOT NULL
 );
 CREATE TABLE Rooms (
   rname VARCHAR(100) PRIMARY KEY
 );
 CREATE TABLE Lectures (
   room VARCHAR(100) NOT NULL
     REFERENCES Rooms,
   course CHAR(6) NOT NULL
     REFERENCES Courses,
   time TIMESTAMP,
   PRIMARY KEY (room, time)
 );
