Handle Null Dates

Use is not null to filter out null dates.

1q = filter q by 'CloseDate' is not null;
2q = foreach q generate 'CloseDate';

Projecting null values does not cause an error. For example, this Date Closed field is empty, but no error occurs.

1q = filter q by year('CloseDate') is null;
2q = foreach q generate year('CloseDate') as "Date Closed";