title: mysql primary key auto increment issue
body: <p>I have a table 
  <code>(entry_id, user_id, event_id)</code>with entry_id set to auto increment. But i had forgotten to make <code>entry_id</code> as primary key. Now i have about 400 entries with same <code>entry_id</code>. It hasnt incremented. How do i change it so that entry_id is unique without deleting the table?</p>
  
  <p>If i do the following query will i lose anything?</p>
  
  <pre><code>ALTER TABLE your_table
  ADD PRIMARY KEY (entry_id);
  </code></pre>
  
  <p>Note that all my <code>entry_id</code>s have 1 as value.</p>