How to add or update row in MySQL if values ​​in columns are duplicate?

Question

PHP

Good evening/afternoon/morning everyone.
We have a table table with somestring, userid, objectid columns.
How can I check in one query if a string with certain values ​​exists (eg userid=2,objectid=34) and if the string is found, update somestring as somestring="ok"?
If such a string does not exist, create it.
The userid column can have a bunch of rows with a value of 2, just like with objectid 34 (many-to-many), but there can't be more than one relationship row 2-34.
Previously, I googled on this issue and found INSERT ON DUPLICATE KEY, but due to the meager knowledge of SQL, this is an overwhelming question for me.
Thank you very much everyone for your help!

Answer 1

add unique index (userid,objectid)
https://dev.mysql.com/doc/refman/5.0/en/insert-on-...