purr_dbplyr_union.R

library(dbplyr, warn.conflicts = FALSE)
library(dplyr, warn.conflicts = FALSE)
library(purrr, warn.conflicts = FALSE)
library(DBI, warn.conflicts = FALSE)
library(rlang, warn.conflicts = FALSE)

con <- DBI::dbConnect(RSQLite::SQLite(), path = ":dbname:")

db_mtcars <- copy_to(con, mtcars)

cyls <- c(4, 6, 8)

all <- cyls %>%
  map(~{
    db_mtcars %>%
      filter(cyl == .x) %>%
      summarise(mpg = mean(mpg, na.rm = TRUE)
      )
  }) %>%
  reduce(function(x, y) union(x, y)) 

all
#> # Source:   lazy query [?? x 1]
#> # Database: sqlite 3.22.0 []
#>     mpg
#>   <dbl>
#> 1  15.1
#> 2  19.7
#> 3  26.7

show_query(all)
#> <SQL>
#> SELECT AVG(`mpg`) AS `mpg`
#> FROM (SELECT *
#> FROM (SELECT *
#> FROM `mtcars`)
#> WHERE (`cyl` = 4.0))
#> UNION
#> SELECT AVG(`mpg`) AS `mpg`
#> FROM (SELECT *
#> FROM (SELECT *
#> FROM `mtcars`)
#> WHERE (`cyl` = 6.0))
#> UNION
#> SELECT AVG(`mpg`) AS `mpg`
#> FROM (SELECT *
#> FROM (SELECT *
#> FROM `mtcars`)
#> WHERE (`cyl` = 8.0))

dbDisconnect(con)