Using Query:

use Northwind
GO

declare @CustIDs varchar(8000)

select top 10
        @CustIDs = isnull(@CustIDs + ',', '') + CustomerID
from    Customers

select  @CustIDs

GO

Using Cursor:

declare cCustomerIDs cursor for

select [CustomerID] from [dbo].[Customers] order by [CustomerID]

declare @CustomerIDs varchar(8000)

declare @CustomerID varchar(10)

open cCustomerIDs

fetch next from cCustomerIDs into @CustomerID

while @@FETCH_STATUS = 0
    begin

        set @CustomerIDs = isnull(@CustomerIDs + ',', '') + @CustomerID

        fetch next from cCustomerIDs into @CustomerID

    end

close cCustomerIDs

deallocate cCustomerIDs

select  @CustomerIDs as CustomerIDs

GO