How to re-use a dynamically created column in a select statement for calculation?

Hi guys,

I was wondering how can I reuse a dynamically created column ("qty" in this case) in a select statement for calculation?

Example as below:

  lt_mat = SELECT m.matnr, m.charg,                  ( SELECT total_qty FROM :lt_quantity                                    WHERE matnr = m.matnr                                       AND werks = m.werks                                      AND lgort = m.lgort                                       AND charg = m.charg ) AS "qty",                   CASE                    WHEN qty > 0                        THEN                            m.clabs - qty                        ELSE                           0                  END AS quant,

I get the error: SQLSCRIPT Message: invalid column name: QTY as a result.

Thanks,

Sam