[datatable-help] merge/join/match

[Gabor Grothendieck]

I am moving this discussion which started with mdowle to the list.

Consider this example slightly modified from the data.table FAQ:

> X = data.table(x=c("a","a","b","b","b","c","c"), foo=1:7, key="x")
> Y = data.table(y=c("b","c","d"), bar=c(4,2,3))
> out <- X[Y]; out
       x foo bar
1:     b   3   4
2:     b   4   4
3:     b   5   4
4:     c   6   2
5:     c   7   2
6:     d  NA   3

Note that the first column of the output is labelled x even though the
data to produce it comes from y, e.g. "d" in out$x is not in X$x but
does appear in Y$y so clearly the data is coming from y as opposed to
x .  In terms of SQL the above would be written:

    select Y.y as x, ...

and the need to renamne the first column of out suggests that there
may be a deeper problem here.

Here are some ideas to address this (they would require changes to data.table):

- the default of X[Y,, match=NA] would be changed to a default of
X[Y,,match=0] so that it corresponds to the defaults in R's merge and
in SQL joins.

- the column name of the first column in the example above would be
changed to y if match=0 but be left at x if match=NA.  In the case
that match=0 (the proposed new default) x and y are equal so the first
column can be validly labelled as x but in the case that match=NA they
are not so y would be used as the column name.

- the name match= does seem a bit misleading since R's match only
matches one item in the target whereas in data.table match matches
many if mult="all" and that is the default.  Perhaps some thought
should be given to a name change here?

The above would seem to correspond more closely to R's merge and SQL
join defaults.  Any use cases or other comments?

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