stevemisc (version 1.8.0)

db_lselect: Lazily select variables from multiple tables in a relational database

Description

db_lselect() allows you to select variables from multiple tables in an SQL database. It returns a lazy query that combines all the variables together into one data frame (as a tibble). The user can choose to run collect() after this query if they see fit.

Usage

db_lselect(.data, connection, vars)

Value

Assuming a particular structure to the database, the function returns a combined table including all the requested variables from all the tables listed in the data character vector. The returned table will have other attributes inherited from how dplyr interfaces with SQL, allowing the user to extract some information about the query (e.g. through show_query()).

Arguments

.data

a character vector of the tables in a relational database

connection

the name of the connection object

vars

the variables (entered as class "character") to select from the tables in the database

Details

This is a wrapper function in which purrr and dplyr are doing the heavy lifting. The tables in the database are declared as a character (or character vector). The variables to select are also declared as a character (or character vector), which are then wrapped in a one_of() function within select() in dplyr.

References

Miller, Steven V. 2020. "Clever Uses of Relational (SQL) Databases to Store Your Wider Data (with Some Assistance from dplyr and purrr)" http://svmiller.com/blog/2020/11/smarter-ways-to-store-your-wide-data-with-sql-magic-purrr/

Examples

# \donttest{
library(DBI)
library(RSQLite)
library(dplyr)
library(dbplyr)
set.seed(8675309)

A <- data.frame(uid = c(1:10),
                a = rnorm(10),
                b = sample(letters, 10),
                c = rbinom(10, 1, .5))

B <- data.frame(uid = c(11:20),
                a = rnorm(10),
                b = sample(letters, 10),
                c = rbinom(10, 1, .5))

C <- data.frame(uid = c(21:30), a = rnorm(10),
                b = sample(letters, 10),
                c = rbinom(10, 1, .5),
                d = rnorm(10))

con <- dbConnect(SQLite(), ":memory:")

copy_to(con, A, "A",
        temporary=FALSE)

copy_to(con, B, "B",
        temporary=FALSE)

copy_to(con, C, "C",
        temporary=FALSE)

# This returns no warning because columns "a" and "b" are in all tables
c("A", "B", "C") %>% db_lselect(con, c("uid", "a", "b"))

# This returns two warnings because column "d" is not in 2 of 3 tables.
# ^ this is by design. It'll inform the user about data availability.
c("A", "B", "C") %>% db_lselect(con, c("uid", "a", "b", "d"))
dbDisconnect(con)
# }