please clarify my doubt

what is the difference between SY-UCOMM & OK_CODE?

and what is the difference between SELECT SINGLE AND SELECT UPTO 1 ROWS? we can mention 'where' condition in both the cases. if we dont mention 'where' condition, by default the 'select single' statement will select 1 row from the database in this case what is the difference. which one of these statements will improve performance? please clarify my doubt.