Recursive Sql Update Query

HI

I need to update a column of a table . if that particular column has any null values then i have to update the column. this should by done one by one. so i wrote a a query which take each row one by one and update

WHILE exists(Select count(projid) From test Where sumVolume  IS NULL)
begin
update a set a.sumVolume12weeks=b.sumVolume12weeks from tatsla_t1_test a
left outer join (
SELECT top 1 CUSTID,PROJID,recvdate,dbo.sumValue(custid,projid,recvdate) as sumVolume
FROM test WHERE sumVolumeIS NULL 
)b on a.custid=b.custid and a.projid=b.projid and a.recvdate=b.recvdate
where a.custid=b.custid and a.projid=b.projid and a.recvdate=b.recvdate 
end