title: referencing multiple tables through a single column in sql
body: <p>In SQL can a single column in a table reference multiple tables?</p>
  
  <p>E.g. if we have tables <code>employee</code> (PK <code>emp_id, name</code>) and <code>customer</code> (PK <code>cust_id, name</code>)</p>
  
  <p>Can we have a table <code>contact</code> (id references <code>[employee, customer], number</code>);</p>
  
  <p>Or do we necessarily need to make 2 tables:</p>
  
  <p><code>contact_cust</code> (<code>cust_id</code> references <code>customer, number</code>) and <code>contact_emp</code> (<code>emp_id</code> references <code>employee, number</code>)</p>
  
  <p>I know that the second choice would be better even if the first one were possible. I just want to know is the first way possible?</p>