title: Increment value by one if value is NULL
body: <p>The query below works if there's at least one row in the table. I want it to return 1 in the event it returns <code>NULL</code>.</p>
  
  <pre><code>mysql&gt; desc users_map_item;
  +----------------+-----------------------+------+-----+---------+-------+
  | Field          | Type                  | Null | Key | Default | Extra |
  +----------------+-----------------------+------+-----+---------+-------+
  | map_user_id    | mediumint(8) unsigned | NO   | MUL | NULL    |       |
  | map_item_id    | mediumint(8) unsigned | NO   | PRI | NULL    |       |
  | unique_item_id | mediumint(8) unsigned | NO   |     | NULL    |       |
  | item_owner     | mediumint(8) unsigned | NO   |     | NULL    |       |
  | privilege      | tinyint(1)            | YES  |     | NULL    |       |
  +----------------+-----------------------+------+-----+---------+-------+
  
  SELECT case when unique_item_id IS NULL then isnull(max(unique_item_id)+1) else     
  max(unique_item_id)+1 end as unique_item_id FROM users_map_item WHERE map_user_id = 1;
  </code></pre>