title: What is the best method to detect duplicate row on mysql and add new values to specific row
body: <p>i am currently working on a site which get video content from other websites.
  My script gets the new videos from categories pages. There are also videos which are listed on 2 or more categories. Over a unique index i can detect when the url is the same but i wished to know how i could get in my category field more then 1 category id. I know how to do this but using php but this would mean i need 3 querys in order to update the field. There would be more than 3 - 4 millions entrys and almost every entry belongs to 2 or 3 diferrent categories.</p>
  
  <p>Is there a way like "ON DUPLICATE KEY add to category_field category id 3"
  so i would end up with a row like: category_field(2,3,100,45,9) only by using mysql AND detect before adding the new category id to the field if the category id already exists in it?</p>
  
  <p>Thank you very much. I hope i could make myself clear enough english is not my native language.</p>