note 2854 deleted from mysql_affected_rows by danbeck

This might seem obvious to some, but this returns the number of rows that have literally been *affected* by the SQL command; i.e., it will return zero if it matches rows but does not change them. For example, if you use an UPDATE command and it matches a record but no values change, this will return zero. If you are using a zero return as an indication of error, this will not make a sensible match for your logic! (You wouldn't programmatically update equal values, but a user might 'update' a record with the values that already existed, which should probably *not* be considered an error condition.)