Got Problem When Combines Tables Using Left Join

Hi,

I'm having problem with left join ...

Let's say i've these following tables

academic
ID | year | level | m_l
----------------------------------------------
147 | 2006 | dip | 20
148 | 2006 | dip | 12


course
ID | year | level | course_desc | m_l
----------------------------------------------
147 | 2006 | dip | k | 7
147 | 2006 | dip | b | 11


Hint
----
(1) see course table, select sum(m_l) from course where ID=147 and year=2006 and level='dip' = 18
(2) after query from (1), if user select ID=147, year=2006, level=dip, then m_l receiving 2 or less only (0,1,2).


What i've done shown as follow:-

SELECT a.ID, a.Year, a.Level, a.m_l - b.m_l [value_left_in_course]
FROM
academic a
LEFT JOIN
(SELECT SUM(m_l) [m_l] FROM course
where ID=147 and year=2006 and level='dip'
GROUP BY ID) b
ON a.ID = b.ID
where a.ID=147 and a.year=2006 and a.level='dip'

the result is
ID | year | level | value_left_in_course
----------------------------------------------
147 | 2006 | dip | 2

MY PROBLEM IS WHEN NO ROWS in course when ID=148, then i'm query as above the result is
ID | year | level | value_left_in_course
----------------------------------------------
147 | 2006 | dip | (null)


How to writing SQL to changing (null), then the result shown as follow?
ID | year | level | value_left_in_course
----------------------------------------------
147 | 2006 | dip | 12