[R] merging dataframes with diffent rownumbers

[Thomas Hoffmann]

Dear R-Helpers,

I have following problem:

I do have two data frames dat1 and dat2 with a commen column BNUM (long 
integer). dat1 has a larger number of BNUM than dat2 and different rows 
of dat2 have equal BNUM. The numbers of rows in dat1 and dat2 is not 
equal.  I applied the  tapply-function to dat2 with BNUM as index. I 
would like to add the columns from dat1 to the results of

b.sum <- tapply(dat2, BNUM, sum).

However the BNUM of b.sum are only a subset of the dat1.

Does anybody knows a elegant way to solve the problem?
Thanks in advance

Thomas H.