SQL & PL/SQL

Hi,

I am supposed to use following statement in query. As per my understanding Sum can not be used inside case statement.
How do I acheive same result.

 
sum(case when sum(a.points) is null then a.transactionamount else sum(a.points) end), 

 
SELECT a.postdateid transaction_date, 
  SUM(a.points) POINTS_OR_MILES_CREDITED, 
  sum(case when sum(a.points) is null then a.transactionamount else sum(a.points) end), 
  FROM accrual a 
join MemberALL m on m.memberid = a.memberid 
WHERE a.awardnumber IS NULL 
GROUP BY a.postdateid 
ORDER BY a.postdateid; 


Thanks 
Sandy