manipulate GNU R data frames using SQL

 The sqldf() function is typically passed a single argument which
 is an SQL select statement where the table names are ordinary R data
 frame names. sqldf() transparently sets up a database, imports the
 data frames into that database, performs the SQL select or other
 statement and returns the result using a heuristic to determine which
 class to assign to each column of the returned data frame. The sqldf()
 or read.csv.sql() functions can also be used to read filtered files
 into R even if the original files are larger than R itself can handle.
 'RSQLite', 'RH2', 'RMySQL' and 'RPostgreSQL' backends are supported.