usage #4
Description
- subset()
http://stackoverflow.com/questions/4605206/drop-columns-r-data-frame - merge() http://stat.ethz.ch/R-manual/R-devel/library/base/html/merge.html
- levels() http://stat.ethz.ch/R-manual/R-patched/library/base/html/levels.html
- faster implementation: http://cran.r-project.org/web/packages/data.table/ (binary search, superkeys)
- http://plyr.had.co.nz/09-user/
- http://stackoverflow.com/questions/1299871/how-to-join-data-frames-in-r-inner-outer-left-right
- https://metacpan.org/pod/Set::Relation
- https://metacpan.org/release/SQL-Statement