A simple upsert is a standalone update operation for which a subsequent standalone insert operation is coded that is performed only if the specific row to be updated is found not to exist. These actions require the performance of two individual SQL requests in the case where no row is found to update. This was the only means of performing a conditional insert on an update failure before the introduction of the upsert form of the UPDATE statement.
The more high-performing atomic upsert operation, represented by the upsert form of the UPDATE statement, is a single SQL request that includes both UPDATE and INSERT functionality. First the operation performs the update. Then, the operation updates the row (if the row exists) or inserts the updated row.