MySQL: Advanced insert if not exists

I have a table with with essentially three columns: user_id, setting, and value. I'm trying to use the following code:

INSERT INTO 'user_settings'(user_id, setting, value)
VALUES (1234, setting_1, 500)
ON DUPLICATE KEY UPDATE user_id = 1234, setting = setting_1'

This works great when creating a new setting, and it doen't generat开发者_如何学运维e duplicate records. The problem comes when I want to change the value- this won't work after the previous query has run:

INSERT INTO 'user_settings'(user_id, setting, value)
VALUES (1234, setting_1, 999)
ON DUPLICATE KEY UPDATE user_id = 1234, setting = setting_1'

No rows are affected. Clearly I'm missing something...

IMPORTANT: I am not able to alter the database (new primary keys or something).

UPDATE: It seems my understanding of ON DUPLICATE KEY is wrong. But the question remains- what is the most efficient way way to accomplish this?

Answered in a comment below: "If the Primary (or Unique) key is (user_id, setting), then use: ... ON DUPLICATE KEY UPDATE value=999".

Assuming you actually have a unique key on user_id, you are getting "no rows affected" because you aren't changing anything in the second query. I think what you want to do is update the value field as well:

INSERT INTO 'user_settings'(user_id, setting, value)
VALUES (1234, setting_1, 999)
ON DUPLICATE KEY UPDATE setting = setting_1,value=999

Without value in there, you're just setting the user_id and the setting field to the same values they were before, and MySQL doesn't need to update the record.

If you don't have a unique key on user_id, you'll have to find a different approach, as the ON DUPLICATE KEY UPDATE won't trigger.