REPLACE
internally performs a delete and then an insert. This can cause problems if you have a foreign key constraint pointing at that row. In this situation the REPLACE
could fail or worse: if your foreign key is set to cascade delete, the REPLACE
will cause rows from other tables to be deleted. This can happen even though the constraint was satisfied both before and after the REPLACE
operation.
Using INSERT ... ON DUPLICATE KEY UPDATE
avoids this problem and is therefore prefered.