sql语句中用sum()函数输出为null?
select count(*) as number,sum(case when signstate=0
then 1 else 0 end) as internet,sum(case when signstate=1 then 1 else 0 end) as
paper
from stusignsys as stu,fdzlist as fdz,studykindlist as st
where stu.fdzid=fdz.fdzid and st.studykindid like 'b%'
and fdz.fdzid='11001' and stu.stu_level='200203' and stu.signdate between '2001-3-3' and '2005-3-5'
and stu.studykindID=st.studykindID
这条语句输出三列,分别是
number internet paper
0 null null
即使查不到数据也应该为0,为什么是null呢?