CDS View with custom fields

Hi,

Need help with CDS view , I'm trying to create a cds view using "union" to combine data from few tables.

Not all fields available from each table, but I still have to declare all fields to keep common view.

DEFINE view zcdsv_union_test as

select from table 1

{ field1 as part,

   field2 as site,

field 3

}

select from table 2

{ field 1 as part,

  field 2 as site,

  0.00   as Qty

In this example,how to declare Qty field in table 2 with certain data type.

Thanks for your help..