I have a QVD file to which I need to add a FLAG (Yes/No).
The rule for the flag is if(Balance) >= 0 or Null than Flag = No and if(Balance) <0 than Flag = Yes.
Assuming that "Balance" is a fieldname in the QVD and "Flag" as the fieldname to be added, try the following code:
LOAD *, If(IsNull(Balance) or Balance >= 0,'No','Yes') as Flag;
LOAD * from Your.qvd (qvd);
Retrieving data ...