All data manipulation on SQL tbls are lazy: they will not actually
run the query or retrieve the data unless you ask for it: they all return
a new tbl_dbi object. Use compute() to run the query and save the
results in a temporary table in the database, or use collect() to retrieve the
results to R. You can see the query with show_query().
Usage
# S3 method for class 'src_dbi'
tbl(src, from, ...)Arguments
- src
A
DBIConnectionobject produced byDBI::dbConnect().- from
Either a table identifier or a literal
sql()string.Use a string to identify a table in the current schema/catalog. We recommend using
I()to identify a table outside the default catalog or schema, e.g.I("schema.table")orI("catalog.schema.table"). You can also usein_schema()/in_catalog()orDBI::Id().- ...
Passed on to
tbl_sql()
Details
For best performance, the database should have an index on the variables
that you are grouping by. Use explain() to check that the database is using
the indexes that you expect.
There is one verb that is not lazy: do() is eager because it must pull
the data into R.
Examples
library(dplyr)
# Connect to a temporary in-memory SQLite database
con <- DBI::dbConnect(RSQLite::SQLite(), ":memory:")
# Add some data
copy_to(con, mtcars)
DBI::dbListTables(con)
#> [1] "mtcars" "sqlite_stat1" "sqlite_stat4"
# To retrieve a single table from a source, use `tbl()`
con %>% tbl("mtcars")
#> # Source: table<`mtcars`> [?? x 11]
#> # Database: sqlite 3.47.1 [:memory:]
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 21 6 160 110 3.9 2.62 16.5 0 1 4 4
#> 2 21 6 160 110 3.9 2.88 17.0 0 1 4 4
#> 3 22.8 4 108 93 3.85 2.32 18.6 1 1 4 1
#> 4 21.4 6 258 110 3.08 3.22 19.4 1 0 3 1
#> 5 18.7 8 360 175 3.15 3.44 17.0 0 0 3 2
#> 6 18.1 6 225 105 2.76 3.46 20.2 1 0 3 1
#> 7 14.3 8 360 245 3.21 3.57 15.8 0 0 3 4
#> 8 24.4 4 147. 62 3.69 3.19 20 1 0 4 2
#> 9 22.8 4 141. 95 3.92 3.15 22.9 1 0 4 2
#> 10 19.2 6 168. 123 3.92 3.44 18.3 1 0 4 4
#> # ℹ more rows
# Use `I()` for qualified table names
con %>% tbl(I("temp.mtcars")) %>% head(1)
#> # Source: SQL [1 x 11]
#> # Database: sqlite 3.47.1 [:memory:]
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 21 6 160 110 3.9 2.62 16.5 0 1 4 4
# You can also use pass raw SQL if you want a more sophisticated query
con %>% tbl(sql("SELECT * FROM mtcars WHERE cyl = 8"))
#> # Source: SQL [?? x 11]
#> # Database: sqlite 3.47.1 [:memory:]
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 18.7 8 360 175 3.15 3.44 17.0 0 0 3 2
#> 2 14.3 8 360 245 3.21 3.57 15.8 0 0 3 4
#> 3 16.4 8 276. 180 3.07 4.07 17.4 0 0 3 3
#> 4 17.3 8 276. 180 3.07 3.73 17.6 0 0 3 3
#> 5 15.2 8 276. 180 3.07 3.78 18 0 0 3 3
#> 6 10.4 8 472 205 2.93 5.25 18.0 0 0 3 4
#> 7 10.4 8 460 215 3 5.42 17.8 0 0 3 4
#> 8 14.7 8 440 230 3.23 5.34 17.4 0 0 3 4
#> 9 15.5 8 318 150 2.76 3.52 16.9 0 0 3 2
#> 10 15.2 8 304 150 3.15 3.44 17.3 0 0 3 2
#> # ℹ more rows
# If you just want a temporary in-memory database, use src_memdb()
src2 <- src_memdb()
# To show off the full features of dplyr's database integration,
# we'll use the Lahman database. lahman_sqlite() takes care of
# creating the database.
if (requireNamespace("Lahman", quietly = TRUE)) {
batting <- copy_to(con, Lahman::Batting)
batting
# Basic data manipulation verbs work in the same way as with a tibble
batting %>% filter(yearID > 2005, G > 130)
batting %>% select(playerID:lgID)
batting %>% arrange(playerID, desc(yearID))
batting %>% summarise(G = mean(G), n = n())
# There are a few exceptions. For example, databases give integer results
# when dividing one integer by another. Multiply by 1 to fix the problem
batting %>%
select(playerID:lgID, AB, R, G) %>%
mutate(
R_per_game1 = R / G,
R_per_game2 = R * 1.0 / G
)
# All operations are lazy: they don't do anything until you request the
# data, either by `print()`ing it (which shows the first ten rows),
# or by `collect()`ing the results locally.
system.time(recent <- filter(batting, yearID > 2010))
system.time(collect(recent))
# You can see the query that dplyr creates with show_query()
batting %>%
filter(G > 0) %>%
group_by(playerID) %>%
summarise(n = n()) %>%
show_query()
}
#> <SQL>
#> SELECT `playerID`, COUNT(*) AS `n`
#> FROM (
#> SELECT `Lahman::Batting`.*
#> FROM `Lahman::Batting`
#> WHERE (`G` > 0.0)
#> ) AS `q01`
#> GROUP BY `playerID`