是否可以直接在类tbl_sql（或tbl_dbi）上运行SQL查询？

Question

Example code:

library(DBI)
library(RSQLite)

# will require more details (like user, password, host, port, etc.)
con <- dbConnect(RSQLite::SQLite(), ":memory:")
data(USArrests)
dbWriteTable(con, "USArrests", USArrests)
dbListTables(con)

d0 <- tbl(con, "USArrests")
dbGetQuery(d0, "select * from USArrests")
dbGetQuery(d0, "select * from d0")

Which returns:

Error in (function (classes, fdef, mtable)  : 
  unable to find an inherited method for function ‘dbGetQuery’ for signature ‘"tbl_dbi", "character"’

Obviously I could just use dbGetQuery on con, but was wondering if there is a way to have it work on d0 directly.

Thanks.

Answer 1

On the one hand, is the tbl() function which is a SQL statement of type SELECT * FROM Table this works over DBMS and to retrieve data from database server it need a pull function like collect() . On the other hand, is the function dbGetQuery() which retrieves the data into R session using a SQL query. Both requires a connection to server and a statement, but the first create the statement using SQL translations and the other is the user which write the SQL query.

To illustrate I will use a temporal table tmp in a postgreSQL DBMS:

# Example on postgreSQL
library(tidyverse)
library(dbplyr)
library(RPostgreSQL)
library(DBI) # This is loaded with RPostgreSQL package

con <- dbConnect(PostgreSQL(), 
                 dbname="test",
                 host="localhost",
                 port=5432,
                 user="user",
                 password="pass")

Dummy data to PostgreSQL server

date <- data_frame(Col1 = c("20180212", "20180213"),
                   Col2 = c("A", "B"))
dbWriteTable(con, "tmp", date, temporary = TRUE)

With tbl() function

tbl(con, "tmp") %>% show_query()

#><SQL> 
#>SELECT * 
#>FROM "tmp"

tbl(con, "tmp") %>% 
  mutate(date = to_date(Col1, "YYYYMMDD")) %>%
  show_query()

#><SQL>
#>SELECT "row.names", "Col1", "Col2", TO_DATE("Col1", 'YYYYMMDD') AS "date"
#>FROM "tmp"

tbl(con, "tmp") %>% 
  mutate(date = to_date(Col1, "YYYYMMDD")) %>% #this works on DBMS
  collect() %>% #This retrive to R session
  str()

#>Classes ‘tbl_df’, ‘tbl’ and 'data.frame': 2 obs. of  3 variables:
#> $ row.names: chr  "1" "2"
#> $ Col1     : chr  "20180212" "20180213"
#> $ Col2     : chr  "A" "B"
#> $ date     : Date, format: "2018-02-12" "2018-02-13"

With dbGetQuery() function

dbGetQuery(con, "SELECT * FROM tmp") %>% 
  str()

#>'data.frame': 2 obs. of  3 variables:
#> $ row.names: chr  "1" "2"
#> $ Col1     : chr  "20180212" "20180213"
#> $ Col2     : chr  "A" "B"

dbGetQuery(con, "SELECT * FROM tmp") %>%
  mutate(date = as.Date(Col1, format = "%Y%m%d")) %>% #This works on R session
  str()

#>'data.frame': 2 obs. of  4 variables:
#> $ row.names: chr  "1" "2"
#> $ Col1     : chr  "20180212" "20180213"
#> $ Col2     : chr  "A" "B"
#> $ date     : Date, format: "2018-02-12" "2018-02-13"

Conclusion

tbl() function is a high level over dbGetQuery() in R-programming. Consider to re-design your code chain understanding the differences between both functions and best uses for these.

Answer 2

No, you can't use dbGetQuery with dplyr in this way, it only work with DBIConnection .

Also, your first query is redundany, d0 already represent the USArrests data and second query is nonsense.

dplyr is using a bit different approach, it uses dplyr verbs and creating an SQL query:

d0 %>% filter(Murder > 10) %>% show_query()

Answer 3

ok, looking at the str helped reveal how to access the needed elements using the following commands:

  con <- d0$src$con
  db_name <- db_list_tables(con)[1]