title: Difference between $this->db->replace() $this->db->update()
body: <p>I don't get the difference between query builder's Replace and Update. Especially the documentation for Replace...</p>
  
  <blockquote>
    <p>This method executes a REPLACE statement, which is basically the SQL standard for (optional) DELETE + INSERT, <strong>using PRIMARY and UNIQUE keys as the determining factor.</strong></p>
  </blockquote>
  
  <p>...but I see no indication of using a PK in the example. Am I missing some fundamental knowledge here? (I understand Update just fine).</p>
  
  <p>Replace</p>
  
  <pre><code>$data = array(
      'title' =&gt; 'My title',
      'name'  =&gt; 'My Name',
      'date'  =&gt; 'My date'
  );
  
  $this-&gt;db-&gt;replace('table', $data);
  </code></pre>
  
  <p>Update</p>
  
  <pre><code>$data = array(
      'title' =&gt; $title,
      'name' =&gt; $name,
      'date' =&gt; $date
  );
  
  $this-&gt;db-&gt;where('id', $id);
  $this-&gt;db-&gt;update('mytable', $data);
  </code></pre>
  
  <p>Thanks.</p>