MySQL 'UPDATE ON DUPLICATE KEY' without a unique column?

Question

What are peoples\' thoughts on the most performance efficient way to do the following query:

• 3 column table

• if the combination of col_

Answer 1

Edit: Ignore my suggestions

You can use a composite key as ceejayoz said, however I think you need REPLACE INTO instead of UPDATE ON DUPLICATE KEY because REPLACE also inserts if no duplicate is found.

Note: I don't know the workings of UPDATE ON DUPLICATE KEY but it sounds like it doesn't perform inserts.