title: FOREIGN KEY doesn't work
body: <p>Can you help me to add foreign keys to my tables?</p>
  
  <p>My tables:</p>
  
  <p><code>_products</code> table:</p>
  
  <pre>
  id_product primary index
  code index
  </pre>
  
  <p><code>_extras</code> table:</p>
  
  <pre>
  id_extra primary index
  code
  name
  </pre>
  
  <p><code>_extra_default</code> table:</p>
  
  <pre>
  id_default primary index
  product
  extra
  </pre>
  
  <p>I tried that, but It doesn't working:</p>
  
  <pre><code>ALTER TABLE _extra_default FOREIGN KEY (product) REFERENCES _products(code)
  </code></pre>