title: MySQL update query with WHERE clause and INNER JOIN not working
body: <p>Can't seem to reach the next step in my update query. I'm able to successfully view columns related to the select no problem:</p>
  
  <pre><code>SELECT sales_flat_order_grid.entity_id,sales_flat_order_grid.increment_id,sales_flat_order.coupon_code
  FROM sales_flat_order_grid 
  INNER JOIN sales_flat_order ON sales_flat_order_grid.entity_id = sales_flat_order.entity_id     
  WHERE sales_flat_order_grid.increment_id = "12345678";
  </code></pre>
  
  <p>This shows 3 columns all where related to the correct increment_id.</p>
  
  <p>The next step is to update the sales_flat_order.coupon_code field. Here is my attempt:</p>
  
  <pre><code>UPDATE sales_flat_order 
  INNER JOIN sales_flat_order ON sales_flat_order_grid.entity_id = sales_flat_order.entity_id      
  WHERE sales_flat_order_grid.increment_id = "12345678"
  SET coupon_code = "newcoupon";
  </code></pre>
  
  <p>But I keep getting a <strong><em>Not unique table/alias: 'sales_flat_order'</em></strong> error message. Could someone point me in the right direction?</p>