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mysql query for current date accounting returns NULL

I have Freeradius 2.x with MySQL 5.5 in Ubuntu. 

I want to query user quota for current date. I am using following code

SELECT (SUM(acctinputoctets)+SUM(acctoutputoctets)) AS Total FROM radacct where (acctstarttime between  DATE_FORMAT(NOW(),'%Y-%m-%d') AND NOW() AND acctstoptime  between  DATE_FORMAT(NOW() ,'%Y-%m-%d') AND NOW()) AND radacct.username='%{User-Name}'

It works fine if there is acctstoptime value in table. but if user have not disconnected yet (and have no previous session for today) it returns NULL.

So how can i can get the value even if user acttstoptime is null?