The solution for problem #100

SELECT advertiser_id, SUM(CASE WHEN AI.ad_id IS NULL THEN 0 ELSE spend END)*1.0/sum(distinct cost) as roi 
FROM advertiser A
LEFT JOIN ad_info AI
ON A.ad_id = AI.ad_id 
GROUP BY advertiser_id

Here is my solution.  The query results are almost same with the official results except one row ( 2.5578947368421048  --- 2.557894736842105)  .  But, it was judged wrong answer.  What is the error?  Thanks.