create table #csv(csvtext varchar(8000) not null)
insert #csv select 'a,b,c,d,e,f,g' union select '1,2,3,4,5,6'

select
	dbo.fnGetCsvPart(csvtext,0,default) as csv_pos0
	,dbo.fnGetCsvPart(csvtext,2,default) as csv_pos2
	,dbo.fnGetCsvPart(csvtext,2,1) as all_csv_from_pos2
from #csv

csv_pos0                       csv_pos2                       all_csv_from_pos2              
------------------------------ ------------------------------ ------------------------------ 
1                              3                              3,4,5,6
a                              c                              c,d,e,f,g


create  function dbo.fnGetCsvPart(@csv varchar(8000),@index tinyint, @last bit = 0)
returns varchar(4000)
as
/* function to retrieve 0 based "column" from csv string */
begin
	declare @i int; set @i = 0
	while 1 = 1
	begin
		if @index = 0
		begin
			if @last = 1 or charindex(',',@csv,@i+1) = 0
				return substring(@csv,@i+1,len(@csv)-@i+1)
			else
				return substring(@csv,@i+1,charindex(',',@csv,@i+1)-@i-1)
		end
		select @index = @index-1, @i = charindex(',',@csv,@i+1)
 		if @i = 0 break
	end
	return null
end
GO