The REPLACE [INTO] syntax allows us to INSERT a row into a table, except that if a UNIQUE KEY (including PRIMARY KEY) violation occurs, the old row is deleted prior to the new INSERT, hence no violation.
Sounds very attractive, and has a nice syntax as well: the same syntax as a normal INSERT INTO’s. It certainly has a nicer syntax then INSERT INTO … ON DUPLICATE KEY UPDATE, and it’s certainly shorter than using a SELECT to see if a row exists, then doing either INSERT or UPDATE.
via code.openark.org » Blog Archive » REPLACE INTO: think twice.