What are practical differences between `REPLACE` and `INSERT … ON DUPLICATE KEY UPDATE` in MySQL?

Question

What I need is to set the values of all the fields of a record with a particular key (the key is composite actually), inserting the record if there is no record with such a

Answer 1

If you don't list all the columns, I think REPLACE will reset any unmentioned columns with their default values in the replaced rows. ON DUPLICATE KEY UPDATE will leave unmentioned columns unchanged.