How to join tables from different SQL databases using R and dplyr?

问题

I'm using dplyr (0.7.0), dbplyr (1.0.0), DBI 0.6-1, and odbc (1.0.1.9000). I would like to do something like the following:

db1 <- DBI::dbConnect(
  odbc::odbc(),
  Driver = "SQL Server",
  Server = "MyServer",
  Database = "DB1"
)
db2 <- DBI::dbConnect(
  odbc::odbc(),
  Driver = "SQL Server",
  Server = "MyServer",
  Database = "DB2"
)
x <- tbl(db1, "Table1") %>%
  dplyr::left_join(tbl(db2, "Table2"), by = "JoinColumn")

but I keep getting an error that doesn't really seem to have any substance to it. When I use show_query it seems like the code is trying to create a SQL query that joins the two tables without taking the separate databases into account. Per the documentation for dplyr::left_join I've also tried:

x <- tbl(db1, "Table1") %>%
      dplyr::left_join(tbl(db2, "Table2"), by = "JoinColumn", copy = TRUE)

But there is no change in the output or error message. Is there a different way to join tables from separate databases on the same server?

回答1:

I'm assuming from the code you provided that (a) you're interested in joining the two tbl objects via dplyr's syntax before you run collect() and pull the results into local memory and that (b) you want to refer directly to the database objects in the call to tbl().

These choices are important if you want to leverage dplyr to programmatically build your query logic while simultaneously leveraging the database server to INNER JOIN large volumes of data down to the set that you're interested in. (Or at least that's why I ended up here.)

The solution I found uses one connection without specifying the database, and spells out the database and schema information using in_schema() (I couldn't find this documented or vignetted anywhere):

conn <- DBI::dbConnect(
  odbc::odbc(),
  Driver = "SQL Server",
  Server = "MyServer"
)

x <- tbl(src_dbi(conn),
         in_schema("DB1.dbo", "Table1")) %>%
  dplyr::left_join(tbl(src_dbi(conn),
                       in_schema("DB1.dbo", "Table2")),
                   by = "JoinColumn")

回答2:

I would use the merge() function to perform the left join the on the tables. It would be something like x <- merge(df1, df2, by = "JoinColumn", all.x = TRUE).

回答3:

I faced the same problem and I wasn't able to solve it with dplyr::left_join.

At least I was able to do the job using the following workaround. I connected to SQL Server without declaring a default database, then I ran the query with sql().

con <- dbConnect(odbc::odbc(), dsn="DWH" ,  uid="", pwd= "" )

data_db <- tbl( con, sql("SELECT * 
                    FROM DB1..Table1 AS a
                    LEFT JOIN DB2..Table2 AS b ON a.JoinColumn = b.JoinColumn") )

data_db %>% ...

Hope it helps.

来源：https://stackoverflow.com/questions/44703608/how-to-join-tables-from-different-sql-databases-using-r-and-dplyr

标签

sql-server

dplyr

dbplyr