[Rd] joining columns as in a relational database

[Prof Brian Ripley]

?merge says

     Merge two data frames by common columns or row names, or do other
     versions of database ``join'' operations.

and it has done all the examples of this sort of thing that I have ever 
needed.

On 24 Jun 2003, Douglas Bates wrote:

> In our recent workshop on "Multilevel Modeling in R" we discussed
> handling data for multilevel modeling.  An classic example of such
> data are test scores of students grouped into schools.  We may wish to
> model the scores as functions of both student-level covariates and
> school-level covariates.
> 
> Such data are often organized in a multi-table format with a separate table
> for each level of information.  The MathAchieve and MathAchSchool data
> frames in the nlme package are examples of such an organization.  The
> HLM software requires the data to be organized like this.  To fit a
> model in R we need to create a composite table by "joining" the
> columns of the student-level and school-level tables, in the
> relational database sense of "join".
> 
> I have created a function to join the columns from two such frames
> according to the values of a key column.  In relational database terms
> the key column must be a primary key for the second frame.  I have
> called this function 'cjoin', by analogy to cbind.
> 
> You can try
> 
> data(MathAchieve, package = 'nlme')
> data(MathAchSchool, package = 'nlme')
> cjoin(MathAchieve, MathAchSchool, "School")
> cjoin(MathAchieve, MathAchSchool, "School", which = "Sector")
> 
> as examples
> 
> Several questions:
> 
>  - Am I duplicating existing functionality?
> 
>  - Is cjoin a good name for such a function?
> 
>  - Would this be useful in base?
> 
> 

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595