count()为空时取值为0

通过count（行数）可以取到0

with
ect1 as
　　(select id from user where is_blacklist=0),
ect2 as
　　(select s1.id,s1.type,s1.date
　　from email s1 where s1.send_id in ect1
　　and s1.receive_id in ect1
　　),
ect3 as
　　(select a.date,count(a.id) as num1,

　　　　<!----以下通过取行数获得cout()为0------>
　　　　(select count(1) from ect2 b
　　　　　　where type='no_completed' and a.date=b.date) as num2
　　from ect2 a
　　group by date)
select date,cast(num2 as FLOAT)/cast(num1 as flaot) from ect3