title: Understanding why Foreign key references to Primary key in the same table?
body: <pre><code>create table employee
   (emp_id smallint unsigned not null auto_increment,
    fname varchar(20) not null,
    lname varchar(20) not null,
    start_date date not null,
    end_date date,
    superior_emp_id smallint unsigned,
    dept_id smallint unsigned,
    title varchar(20),
    assigned_branch_id smallint unsigned,
    constraint fk_e_emp_id 
      foreign key (superior_emp_id) references employee (emp_id),
    constraint fk_dept_id
      foreign key (dept_id) references department (dept_id),
    constraint fk_e_branch_id
      foreign key (assigned_branch_id) references branch (branch_id),
    constraint pk_employee primary key (emp_id)
   );
  </code></pre>
  
  <p>I'm studying this <a href="http://examples.oreilly.com/9780596007270/LearningSQLExample.sql" rel="nofollow">Example</a> and i noticed in <code>employee table</code></p>
  
  <pre><code>emp_id is primary key 
  </code></pre>
  
  <p>and <code>superior_emp_id which is a foreign key</code> references to the <code>emp_id</code> in the same table </p>
  
  <p>creating foreign key which references to primary key in the same table why such a database design can any one help me ?</p>