将Null转换为0

Question

我正在对我的数据库运行此查询，并且它在脚本上按月和周返回null;但是，当我为年份总计运行时，不会返回null。

一年中的那个星期和月份不起作用的是什么？我想我完全拙劣的代码，但无法找到。

周：

declare @today datetime = '2015-10-15' 
set nocount on; 

select [Week Total] = convert(decimal(18,2), sum(isnull([Total],0)))
from [Accounting].[dbo].[Pay] 
where [AccountingDate] <= @today and 
[AccountingDate] > dateadd(week, -1, @today);

月：

declare @today datetime = '2015-10-15'
set nocount on; 

select [Month Total] = convert(decimal(18,2), sum(isnull([Total],0)))
from [Accounting].[dbo].[Pay] 
where [AccountingDate] <= @today and 
[AccountingDate] > dateadd(month, -1, @today);

年份：

declare @today datetime = '2015-10-15'
set nocount on;

select [Year Total] = convert(decimal(18,2), sum(isnull([Total],0)))
from [Accounting].[dbo].[Pay]
where [AccountingDate] <= @today and
[AccountingDate] > dateadd(year, -1, @today);

进行更改以显示一致性。