第一種：
select sum(decode(count1,null,0,count1) +decode(count2,null,0,count2) +decode(count3,null,0,count3)) a from test_table
第二種：
select sum (case when count1 is not null then count1 when count2 is not null then count2 when count3 is not null then count3 else '0' end ) a from test_table