title: MySql check if foreign key constraint exists
body: <p>If I have:</p>
  
  <pre><code>create table order_items
  ( 
    orderid int unsigned not null references orders(orderid),
    isbn char(13) not null,
    quantity tinyint unsigned,
  
    primary key (orderid, isbn)
  
  );
  </code></pre>
  
  <p>How can I check if <code>orderid int unsigned not null references orders(orderid)</code> foreign key exists?</p>