For what it's worth, if we define Rsql as the following script:

  #!/usr/bin/env Rscript
  library(sqldf, quietly=TRUE)
  statement <- commandArgs(TRUE)
  a <- read.table(file("stdin"), header=TRUE)
  print(sqldf(statement))

then we can call

  ps -Ao user | Rsql 'select USER, count(*) as nprocesses from a group by USER limit 5'

to get the output

          USER nprocesses
  1      avahi          2
  2     colord          1
  3     daemon          1
  4     marcle        136
  5 messagebus          1