Question : how to use the results of a sql sub-select query in a where statement

I am creating a sql view and one of the columns I need comes from a different table that I can't join to.  I am using a sub-select, within the query to get the data.  The sub-select is as follows:
,CAST ((SELECT     InvoiceNumber
            FROM         Invoicedata id
            WHERE     (id.ShipNum  = sl.ShipNum)) AS Varchar(100))
            AS Invoice
In a where clause, I am trying to return records where the Invoice is NULL.  When I put the column Invoice into the where clause, it tells me Invoice is an invalid column name.  Is there a way to use the Invoice column in the where clause?
Thanks in advance, JoMar

Answer : how to use the results of a sql sub-select query in a where statement

I would use a repater, which offers better customization of the output
yes you would need a nested repeater

here a simple tutorial on how to manipulate repeaters
http://msdn.microsoft.com/en-us/library/aa478959.aspx