Question

When using RPostgreSQL I find that I cannot use sqldf in the same way. For example if I load the library and read in data into a data frame using the following code:

library(RPostgreSQL)
drv <- dbDriver("PostgreSQL")
con <- dbConnect(drv, host="localhost", user="postgres", password="xxx", dbname="yyy", port="5436")
rs <- dbSendQuery(con, "select * from table");                           
df<- fetch(rs, n = -1); dbClearResult(rs) 
dbDisconnect(con) 

I know have the contents of this table in the dataframe df. However if I want to run a SQL command using sqldf I would previously do something like this:

sqldf("SELECT * FROM df WHERE X > 10")

This no longer works as I get the error message:

Error in postgresqlNewConnection(drv, ...) : 
  RS-DBI driver: (could not connect postgres@localhost on dbname "test"
)
Error in !dbPreExists : invalid argument type

I assume this is operator error on my part, but I can’t figure how what arguments to supply to sqldf so that it just focuses on the data frame and does not try to connect to anything else.

Answer 1

Using sqldf with RPostgreSQL

sqldf will automatically work with the test database in PostgreSQL if it sees that RPostgreSQL is loaded. So you can create a test database in PostgreSQL and then use sqldf with that

or, you can specify the name of a different database.

See: sqldf FAQ 12

Using sqldf with RSQLite

If you want to use sqldf with RSQLite rather than with RPostgreSQL you can use sqldf‘s drv argument to force it use a non-default driver. e.g.

sqldf("select foo from bar...",drv="SQLite")

or, you can set the driver globally using the "sqldf.driver" option. From within R:

options(sqldf.driver = "SQLite")

or, another possibility if you wish to use RSQLite is to detach RPostgreSQL before you use sqldf and load it again afterwards.

See ?sqldf for details.