[R] R is not for Relational

[Robert Gentleman]

On Tue, Aug 03, 1999 at 10:44:17PM +0200, Peter Dalgaard BSA wrote:
> Russell Senior <seniorr at teleport.com> writes:
> 
> > Peter> Check if subset() does something in the right direction. What
> > Peter> would the select statement do, precisely? (I don't speak SQL)
> > 
> > One useful thing that subset() doesn't appear to do is joins.  That is
> > frequently what select statements are used for in SQL.  There are
> > three S-plus functions (at least, as documented in MASS 2ed) called
> > merge, by and aggregate.  The merge() function joins data frames, but
> > is not implemented in R that I am aware of, at least as of 0.64.0.
> 
> Hmm. What does the SQL syntax (and semantics) look like for that kind
> of operation? Would it make sense to try and turn the existing
> subset() into a select() function which could do joins as well?
> 
Join in SQL parlance is a way to join two (or more tables).
A very simple example is:

	SELECT XX, XY, YX, YY
	 FROM TABX, TABY
	WHERE XG = YG

  If we imagine that TABX has the X variables and TABY has the Y ones
then the result should be a table with 4 columns, XX and XY from TABX,
 YX and YY from TABY. It will have one row for each XG that matches
an element in YG.
  I think that Splus's merge does something a bit different but I
can't be sure without trying it out.
  This is of course a very simple example; (they seem to call it an
equi-join in some circles) and things can get a lot more complicated.

+-------------------------------------------------------------------------+
| Robert Gentleman              voice: (617) 632-5045                     |
| Senior Lecturer               fax:   (617) 632-2444                     |
| University of Auckland	email1: rgentlem at jimmy.harvard.edu        |
| on leave at:                                                            |
| Dana-Farber Cancer Institute  email2: rgentlem at stat.auckland.ac.nz      |
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