alter function fn_getStr(@v int)
returns varchar(32)
begin
    declare @retValue varchar(32)
    set @retValue =''
    if (@v is not null)
    
    begin
        
        declare @s varchar(16),
                
                @l tinyint,
                @i tinyint
        set @i = 1
        set @s =convert(varchar(16),@v)
        set @l = len(@s)
        while(@i<=@l)
        begin
            set @retValue = @retValue + substring(@s,@i,1)+','
            --print @retValue
            set @i = @i + 1
        end
    end
    return left(@retValue,len(@retValue)-1)
end

select dbo.fn_getStr(123)

--------------------------------
1,2,3,4

(1 行受影响)

------解决方案--------------------
SQL code
--> 测试数据: #tb
if object_id('tempdb.dbo.#tb') is not null drop table #tb
create table #tb ([val] varchar(10))
set nocount on
insert into #tb
select '1234'

declare @Num table(id int)
insert into @Num select number from master..spt_values where number between 1 and 100 and type='p'
set nocount off

declare @str varchar(100)
select @str=isnull(@str+','+element,element) from (
select SUBSTRING(val,id,1) element from #tb a 
join @Num b on SUBSTRING(val,id,1)<>'') as T 
print(@str)
--结果
/*
1,2,3,4
*/