You've to use LEFT OUTER JOIN. Also, it's not a good idea to use one letter table aliases because it makes code hard to read and could conflict with work area names A-J.

select rs.callno, rs.reservee, NVL(br.borrower, "Unknown") as borrower ;
	from reserve rs 
		LEFT JOIN borrower br ON rs.callno = br.callno

>select a.callno, a.reservee, b.callno from reserve a, borrower b where a.callno=b.callno

>
>how can i use SELECT-SQL to select all records from table reserve even if it does not satisfy the WHERE condition?
>