SQL Server 2008 变量叠加必须初始化

在使用存储过程中的变量(里面有变量),不返回结果集。经过调试,发现变量为null。初始化就OK了,示例如下
declare @num int ,@bb nvarchar(2000), @cc nvarchar(2000),@rm varchar(5) ,@sqls nvarchar(2000)
set @num=1
set @bb='' --不赋值的话,最后叠加为null
set @cc='' 
while @num<=CONVERT(int,'02')
	begin
	if @num<>1
		begin
			set @cc+=N' round(AVG(case when rmonth<='+convert(varchar,@num)+' then convert(float, radio) else null end ),1) [1~'+convert(varchar,@num)+'月份],'
		end
	set @bb+=N' max(case when rmonth='+convert(varchar,@num)+' then radio else 0 end) ['+convert(varchar,@num)+'月份],'	
	set @num=@num+1
	end
	 
	 set @sqls=N'select acity [考核城市],'  
if LEN(@cc)>0
	begin
		set @sqls+=@bb+left(@cc,LEN(@cc)-1)
	end
else
	begin
		set @sqls+=left(@bb,LEN(@bb)-1)
	end
set @sqls+=' from #temp group by acity,torder'
	exec sp_executesql @sqls
	print @sqls

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