Conditional UPDATE in MySQL

I am trying to UPDATE values from a table, but I need to add some conditions. I found the function CASE, but I am not if it is the best method.

Here is an example. My table is ‘relation’:

userid1 | userid2 | name1 | name2

I got for example:

SELECT * 
FROM relation 
WHERE (userid1 = 3 AND userid2 = 4)
      OR (userid1 = 4 AND userid2 = 3);  

Output:

4 | 3 | bill | jack

and I want to change the name of the user 3 in the relation between 3 and 4, but I don’t know if it is the userid1 or the userid2.

I thought of case:

UPDATE relation 
   CASE WHEN userid1 = 3 THEN SET name1 = 'jack' END 
        WHEN userid2 = 3 THEN SET name2 = 'jack' END 
WHERE (userid1 = 3 AND userid2 = 4) 
      OR (userid1 = 4 AND userid2 = 3);

But it doesn’t work! Any ideas?

Thanks very much in advance.

Answer

Unfortunately it’s not very clear what you want to get in the end, but here is how you could correctly use conditional SET in your UPDATE

UPDATE relation 
   SET name1 = CASE WHEN userid1 = 3 THEN 'jack' ELSE name1 END,
       name2 = CASE WHEN userid2 = 3 THEN 'jack' ELSE name2 END
WHERE (userid1 = 3 AND userid2 = 4) 
   OR (userid1 = 4 AND userid2 = 3);

Here is SQLFiddle demo.