create function fn_find(@find varchar(8000), @str varchar(8000), @n smallint)

    returns int

as

begin

    if @n < 1 return (0)

    declare @start smallint, @count smallint, @index smallint, @len smallint

    set @index = charindex(@find, @str)

    if @index = 0 return (0)

    else select @count = 1, @len = len(@find)

    while @index > 0 and @count < @n

        begin

            set @start = @index + @len

            select @index = charindex(@find, @str, @start), @count = @count + 1

        end

    if @count < @n set @index = 0

    return (@index)

end

go  

declare @str varchar(100)

set @str='A,B,C,D,A,B,C,D,C,D,B,A,C,E'  

select dbo.fn_find('A',@str,1) as one, dbo.fn_find('A',@str,2) as two, dbo.fn_find('A',@str,3) as three, dbo.fn_find('A',@str,4) as four

/*

one         two         three       four

----------- ----------- ----------- -----------

1           9           23          0

*/