title: mysql insert on duplicate FIELD instead of KEY
body: <pre>-------------------------------------
  | user_id | user_name | user_visits |
  -------------------------------------
  | 1       | foo       | 5           |
  -------------------------------------
  | 2       | bar       | 12          |
  -------------------------------------</pre>
  
  <p><b>user_id: auto increament, user_visits: default 1</b></p>
  
  <p>INSERT INTO table (user_name) VALUES ('baz'), ('bar'), ('qux');</p>
  
  <p>the above statement will of course insert 3 new records, as the result:</p>
  
  <pre>-------------------------------------
  | user_id | user_name | user_visits |
  -------------------------------------
  | 1       | foo       | 5           |
  -------------------------------------
  | 2       | bar       | 12          |
  -------------------------------------
  | 3       | baz       | 1           |
  -------------------------------------
  | 4       | bar       | 1           |
  -------------------------------------
  | 5       | qux       | 1           |
  -------------------------------------</pre>
  
  <p>but what I'm trying to achieve is:</p>
  
  <pre>-------------------------------------
  | user_id | user_name | user_visits |
  -------------------------------------
  | 1       | foo       | 5           |
  -------------------------------------
  | 2       | bar       | 13          |
  -------------------------------------
  | 3       | baz       | 1           |
  -------------------------------------
  | 4       | qux       | 1           |
  -------------------------------------</pre>
  
  <p>so literally,</p>
  
  <p>if field user_name exists, update user_visits, else insert a new record.</p>
  
  <p>is it possible to achieve this with a single insert statement?</p>