title: PostgreSQL error : there is no unique constraint matching given keys for referenced table
body: <pre><code>create table instructor(
  instructor_id varchar(15) not null,
  instructor_name varchar(15) not null,
  dept_name varchar(15) not null,
  primary key(instructor_id),
  unique(instructor_name));
  
  create table time_slot(
  time_slot_id varchar(15) not null,
  day varchar(15) not null,
  start_time varchar(15) not null,
  end_time varchar(15) not null,
  primary key(time_slot_id, day, start_time));
  
  create table student(
  student_id varchar(15) not null,
  user_id varchar(15) not null,
  password varchar(15) not null,
  num_item int,
  primary key(student_id, user_id));
  
  
  create table section(
  course_id varchar(15) not null,
  sec_id varchar(15) not null,
  semester varchar(15) not null,
  year varchar(15) not null,
  title varchar(15) not null,
  building varchar(15),
  room_no varchar(15),
  time_slot_id varchar(15),
  dept_name varchar(15),
  credits varchar(15),
  instructor_id varchar(15) not null,
  primary key(course_id, sec_id, semester, year),
  foreign key(time_slot_id) references time_slot(time_slot_id),
  foreign key(instructor_id) references instructor(instructor_id));
  
  create table evaluation(
  user_id varchar(15) not null,
  sec_id varchar(15) not null,
  total_score int not null,
  workload int not null,
  difficulty int not null,
  attendance int not null,
  grade int not null,
  accomplishment int not null,
  comment varchar(200),
  primary key(user_id, sec_id),
  foreign key(user_id) references student(user_id),
  foreign key(sec_id) references section(sec_id));
  
  
  create table teaches(
  instructor_id varchar(15) not null,
  course_id varchar(15) not null,
  sec_id varchar(15) not null,
  semester varchar(15) not null,
  year varchar(15) not null,
  primary key(instructor_id, course_id, sec_id, semester, year),
  foreign key(instructor_id) references instructor(instructor_id),
  foreign key(course_id) references section(course_id),
  foreign key(sec_id) references section(sec_id),
  foreign key(semester) references section(semester),
  foreign key(year) references section(year));
  </code></pre>
  
  <p>I tried to create tables using postgresql with these codes, but it's keep saying that there is no unique constraint matching given keys for referenced table. This message comes up when I tried to create table teaches, section, and evaluation.</p>
  
  <p>Please help me. </p>
  
  <p>+) I changed the order of create statement, and added unique key in table instructor, but still not working. </p>