mysql左接続とcase when使い方
SELECT
t1.id,
t1.channel_id,
t1.sum_amount,
t1.sum_date,
t1.msisdn_num,
t2.num,
t2.id AS channel_complaint_id,
case
when t2.num is not null then t2.num/(t1.msisdn_num*100)
else null end scale
FROM
ddo_channel_statistics_day t1
LEFT JOIN ddo_channel_complaint_day t2 ON t1.channel_id = t2.channel_id
AND t1.sum_date = t2.sum_date