Submitted by: Matt Dowle; Assigned to: Nobody; [R-Forge link](https://r-forge.r-project.org/tracker/index.php?func=detail&aid=2108&group_id=240&atid=5356) With details from comments here : http://stackoverflow.com/questions/11233183/grouping-in-data-table-how-to-get-more-than-1-column-of-results