You can use this query itself in qlikview. Before tat, Connect ur db to qlikview via ODBC or oledb connection, then use it..
Select <select_lisr> from tableA A left join tableB B on A.key = B.Key where B.Key is null;
Then store it in the qvd for better performance.
Next time , instead of fetch data from db, use qvd file for better performance