title: In PostgreSQL does a foreign key constraint only require REFERENCES?
body: <p>I was reading the <a href="http://www.postgresql.org/docs/9.3/static/ddl-constraints.html" rel="nofollow noreferrer">docs</a> on PostgreSQL constraints because I wanted to see how to define foreign keys. In their examples</p>
  
  <pre><code>CREATE TABLE orders (
      order_id integer PRIMARY KEY,
      product_no integer REFERENCES products (product_no),
      quantity integer
  );
  </code></pre>
  
  <p>I don't see <code>FOREIGN KEY</code> anywhere; however, in several other stack overflow questions (<a href="https://stackoverflow.com/questions/10356484/how-to-add-on-delete-cascade-constraints">How to add &quot;on delete cascade&quot; constraints?</a> for example) I have seen the <code>FOREIGN KEY</code> written. Is it necessary to write <code>FOREIGN KEY</code> or is it only necessary to use <code>REFERENCES</code>?</p>