问SQL:如何使用条件聚合值(CASE WHEN？)

EN

 select  user_id, sum(a.transaction_amount + b.transaction_amount).
 from my_table as a
 inner join  my_table as b on date_add(b.transaction_date, interval 1 day) =  a.transaction_date
 where date(a.transaction_date) =  STR_TO_DATE('31,8,2016','%d,%m,%Y')
 and a.transaction_amount > 0
 group by user_id

SELECT User_ID, SUM(Transaction_Amount)
FROM yourTable
WHERE DATE(Transaction_Date) = '2016-08-30'
  OR (DATE(Transaction_Date) = '2016-08-31' AND Transaction_Amount > 0)
GROUP BY User_ID

select userid,
       sum(case when date = '2016-08-30' then amount else 0 end) as amt_20160830,
       sum(case when date = '2016-08-31' then amount else 0 end) as amt_20160831
from t
group by userid
having sum(case when date = '2016-08-31' then amount else 0 end) > 0;