if you have a field which contains only the firms you want to keep in the records or which firms you dont want in the records you can work with exists/not exists
Modify your script the following way:
1. Remove Sony and Samsung from Mapping
2. within applymap set Null() as default value (if you want to use exists, otherwise pass original field as default value)
MAPPING LOAD * INLINE [
ApplyMap('DLMap', DL_DESC, Null()) as DL_DESC
xxxxx where exists (DL_DESC, ApplyMap('DLMap', DL_DESC, Null()) ;
use the field which contains all records you want to keep in the exists clause. If you have a field containing the records you want to exclude use "not exits( Field, ApplyMap('DLMap', DL_DESC, Null())" instead.