--> 测试数据：[month]
if object_id('[month]') is not null drop table [month]
create table [month]([year] int,[month] int,[value] numeric(2,1))
insert [month]
select 2011,1,-1.2 union all
select 2011,2,-1.2 union all
select 2011,3,-0.9 union all
select 2011,4,-0.9 union all
select 2011,5,-0.7 union all
select 2011,6,-0.8 union all
select 2011,7,-1.1 union all
select 2011,8,-1.0 union all
select 2011,9,-1.0 union all
select 2011,10,-0.8 union all
select 2011,11,-0.3 union all
select 2011,12,-0.2
--> 测试数据：[day]
if object_id('[day]') is not null drop table [day]
create table [day]([year] int,[month] int,[value] int,[C4] numeric(2,1))
insert [day]
select 2011,1,1,-0.7 union all
select 2011,1,2,-0.2 union all
select 2011,1,3,-0.8 union all
select 2011,1,4,-0.5 union all
select 2011,1,5,-1.2 union all
select 2011,1,6,-1.3 union all
select 2011,1,7,-1.4 union all
select 2011,1,8,-0.8 union all
select 2011,1,9,-1.2 union all
select 2011,1,10,-2.1 union all
select 2011,1,11,-1.4 union all
select 2011,1,12,-1.2 union all
select 2011,1,13,-1.6


select a.[year],a.[month],a.value as [day],a.C4-a.valueb as daymonth
from (
select a.*,b.value as valueb from [day] a inner join [month] b
on a.[year]=b.[year] and a.[month]=b.[month]) a
/*
year    month    day    daymonth
2011    1    1    0.5
2011    1    2    1.0
2011    1    3    0.4
2011    1    4    0.7
2011    1    5    0.0
2011    1    6    -0.1
2011    1    7    -0.2
2011    1    8    0.4
2011    1    9    0.0
2011    1    10    -0.9
2011    1    11    -0.2
2011    1    12    0.0
2011    1    13    -0.4
*/