mysql - best way to provide unique voucher

I am designing a website that will provide vouchers (serial codes). All voucher codes generated and saved in a table like this and not generated online: v_id voucher status (used?) ------------------------------------------------------- 1 abcdefgabcdefg1 0 2 abcdefgabcdefg2 0 3 abcdefgabcdefg3 0 4 abcdefgabcdefg4 0 I should display unused (status:0) voucher to end user after successful payment but I am not sure how can I do this because this site may have high traffic so more than one people may visit site in same milisecond of time, so for example I can not just use: SELECT * FROM table WHERE status=0 LIMIT 0,1; and then UPDATE it: UPDATE table SET status=1 WHERE v_id=.... because two people may visit in same time so we provide same serial code in SELECT before we UPDATE. what would be best way to ensure I display unique serial code?