title: mysql, insert with select sub query
body: <p>I have lots of products in a <code>product_category</code> table with a <code>category_id</code> of 5.
  I have added a new <code>category_id</code> of 19.
  How do I add all products in category 5 to category 19 as well?</p>
  
  <p>I am trying to do something like this:</p>
  
  <pre><code>insert into product_categories( category_id, product_id )
  select 19,
     (select product_id
      from product_categories
      where category_id = 5)
  </code></pre>
  
  <p>but I am getting a <code>sub query returns more than 1 row</code> error.</p>