Tom Schenk Jr wrote:
I use RODBC as my conduit from R to SQL. It works well when the tables are
stored on one channel, e.g.,
channel <- odbcConnect("data_base_01", uid="....", dsn="....")
However, I often need to match tables across multiple databases, e.g.,
"data_base_01" and "data_base_02". However, odbcConnect() appears limited
insofar as you may only query from tables within a single channel, e.g.,
database. I do not have access to write and create new tables on the SQL
servers, which is a possible solution (e.g., copy all tables into a single
database).
Is there any way, in RODBC or another R-friendly SQL package, to perform SQL
operations across multiple databases?
I'm not sure if this can be done with odbc, but with MySQL it is
possible to do joins across multiple databases, and creating temporary
tables may be possible even without the write access you would need for
a permanent table. I'm not sure if you can pass this kind of statement
from R, because the connection usually specifies the database. However,
I have constructed temporary tables with a simple mysql client and then
queried them from R. They stay around as long as you don't quit the
simple client. I am not really sure this is suppose to work. Another
option is two connections and do some of the comparison in R, or write
the results to an SQLite connection, on which you usually have write
access. This might be slow and you may have to deal with chunks if you
have big tables. Joins across databases are also possible with
PostgreSQL, I'm told, but they are more difficult.
Paul
Warm regards.
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