This package abstracts typical patterns used when connecting to and retrieving data from databases in R. It aims to provide very few, simple and reliable functions for sending queries and data to databases.
devtools::install_github("INWT/dbtools")For basic usage consider the simple case where we want to retrieve some data from a SQLite database. At this time we only have sendQuery and no sendData so we use the standard example for setting up the database:
library("RSQLite")
con <- dbConnect(SQLite(), "example.db")
USArrests$State <- rownames(USArrests)
dbWriteTable(con, "USArrests", USArrests, row.names = FALSE)
dbDisconnect(con)This will create a database example.db to which we can send some queries. To begin with, we have to define an object of class Credentials which will store all necessary information to connect to a database. The driver is mandatory, all other arguments depend on the specific back-end.
library("dbtools")
cred <- Credentials(drv = RSQLite::SQLite, dbname = "example.db")
testConnection(cred)## INFO [2016-12-02 11:57:36] example.db OK
cred## An object of class "Credentials"
## drv:SQLiteDriver
## dbname: example.db
Opposed to the dbSendQuery function available from DBI, sendQuery needs a Credentials instance as argument and will take care of connecting to the database, fetching the results and closing the connection.
dat <- sendQuery(cred, "SELECT * FROM USArrests;")
dat## # A tibble: 50 × 5
## Murder Assault UrbanPop Rape State
## <dbl> <int> <int> <dbl> <chr>
## 1 13.2 236 58 21.2 Alabama
## 2 10.0 263 48 44.5 Alaska
## 3 8.1 294 80 31.0 Arizona
## 4 8.8 190 50 19.5 Arkansas
## 5 9.0 276 91 40.6 California
## 6 7.9 204 78 38.7 Colorado
## 7 3.3 110 77 11.1 Connecticut
## 8 5.9 238 72 15.8 Delaware
## 9 15.4 335 80 31.9 Florida
## 10 17.4 211 60 25.8 Georgia
## # ... with 40 more rows
In your normal work-flow you will sometimes want to split up a complex query into more tangible chunks. The approach we take here is to allow for a vector of queries as argument. The result of these queries have to be row-bindable. To make an example lets say we want to query each state separately:
queryFun <- function(state) {
paste0("SELECT * FROM USArrests WHERE State = '", state, "';")
}
sendQuery(cred, queryFun(dat$State))## # A tibble: 50 × 5
## Murder Assault UrbanPop Rape State
## <dbl> <int> <int> <dbl> <chr>
## 1 13.2 236 58 21.2 Alabama
## 2 10.0 263 48 44.5 Alaska
## 3 8.1 294 80 31.0 Arizona
## 4 8.8 190 50 19.5 Arkansas
## 5 9.0 276 91 40.6 California
## 6 7.9 204 78 38.7 Colorado
## 7 3.3 110 77 11.1 Connecticut
## 8 5.9 238 72 15.8 Delaware
## 9 15.4 335 80 31.9 Florida
## 10 17.4 211 60 25.8 Georgia
## # ... with 40 more rows
In such a case sendQuery will perform all queries on one connection. A different approach is to fetch the results of the original query in chunks, which we do not support yet.
One of the problems we face on a regular basis are connection problems to external servers. To address this sendQuery will evaluate everything in a 'try-catch' handler abstracted in dbtools::reTry. With this you can state how many tries a query has, how many seconds should be waited between each iteration and how the error messages should be logged:
dat <- sendQuery(
cred,
"SELECT * FROM USArrest;", # wrong name for illustration
tries = 2,
intSleep = 1
)## ERROR [2016-12-02 11:57:36] Error in rsqlite_send_query(conn@ptr, statement) :
## no such table: USArrest
##
## ERROR [2016-12-02 11:57:37] Error in rsqlite_send_query(conn@ptr, statement) :
## no such table: USArrest
## Error in reTry(function(...) lapply(query, . %>% sendQuery(db = con, ...)), : Error in rsqlite_send_query(conn@ptr, statement) :
## no such table: USArrest
Sometimes your data can be distributed on different servers but you want to send the same query to those servers. What you can do is give sendQuery a CredentialsList.
file.copy("example.db", "example1.db")Now we want to load the data from example1.db and example.db which can be implemented as follows:
cred <- Credentials(
RSQLite::SQLite,
dbname = c("example.db", "example1.db")
)
sendQuery(cred, "SELECT * FROM USArrests;")## # A tibble: 100 × 5
## Murder Assault UrbanPop Rape State
## <dbl> <int> <int> <dbl> <chr>
## 1 13.2 236 58 21.2 Alabama
## 2 10.0 263 48 44.5 Alaska
## 3 8.1 294 80 31.0 Arizona
## 4 8.8 190 50 19.5 Arkansas
## 5 9.0 276 91 40.6 California
## 6 7.9 204 78 38.7 Colorado
## 7 3.3 110 77 11.1 Connecticut
## 8 5.9 238 72 15.8 Delaware
## 9 15.4 335 80 31.9 Florida
## 10 17.4 211 60 25.8 Georgia
## # ... with 90 more rows
It might also be of interest to query your databases in parallel. For that it is possible to supply a apply/map function which in turn can be a parallel lapply like mclapply or something else:
sendQuery(
cred,
"SELECT * FROM USArrests;",
mc.cores = 2,
applyFun = parallel::mclapply
)## # A tibble: 100 × 5
## Murder Assault UrbanPop Rape State
## <dbl> <int> <int> <dbl> <chr>
## 1 13.2 236 58 21.2 Alabama
## 2 10.0 263 48 44.5 Alaska
## 3 8.1 294 80 31.0 Arizona
## 4 8.8 190 50 19.5 Arkansas
## 5 9.0 276 91 40.6 California
## 6 7.9 204 78 38.7 Colorado
## 7 3.3 110 77 11.1 Connecticut
## 8 5.9 238 72 15.8 Delaware
## 9 15.4 335 80 31.9 Florida
## 10 17.4 211 60 25.8 Georgia
## # ... with 90 more rows
Potentially you can send multiple queries to multiple databases. The results are tried to be simplified by default:
sendQuery(cred, c("SELECT * FROM USArrests;", "SELECT 1 AS x;"))## [[1]]
## # A tibble: 100 × 5
## Murder Assault UrbanPop Rape State
## <dbl> <int> <int> <dbl> <chr>
## 1 13.2 236 58 21.2 Alabama
## 2 10.0 263 48 44.5 Alaska
## 3 8.1 294 80 31.0 Arizona
## 4 8.8 190 50 19.5 Arkansas
## 5 9.0 276 91 40.6 California
## 6 7.9 204 78 38.7 Colorado
## 7 3.3 110 77 11.1 Connecticut
## 8 5.9 238 72 15.8 Delaware
## 9 15.4 335 80 31.9 Florida
## 10 17.4 211 60 25.8 Georgia
## # ... with 90 more rows
##
## [[2]]
## # A tibble: 2 × 1
## x
## <int>
## 1 1
## 2 1
sendQuery(cred, c("SELECT * FROM USArrests;", "SELECT 1 AS x;"), simplify = FALSE)## [[1]]
## [[1]][[1]]
## # A tibble: 50 × 5
## Murder Assault UrbanPop Rape State
## <dbl> <int> <int> <dbl> <chr>
## 1 13.2 236 58 21.2 Alabama
## 2 10.0 263 48 44.5 Alaska
## 3 8.1 294 80 31.0 Arizona
## 4 8.8 190 50 19.5 Arkansas
## 5 9.0 276 91 40.6 California
## 6 7.9 204 78 38.7 Colorado
## 7 3.3 110 77 11.1 Connecticut
## 8 5.9 238 72 15.8 Delaware
## 9 15.4 335 80 31.9 Florida
## 10 17.4 211 60 25.8 Georgia
## # ... with 40 more rows
##
## [[1]][[2]]
## # A tibble: 1 × 1
## x
## <int>
## 1 1
##
##
## [[2]]
## [[2]][[1]]
## # A tibble: 50 × 5
## Murder Assault UrbanPop Rape State
## <dbl> <int> <int> <dbl> <chr>
## 1 13.2 236 58 21.2 Alabama
## 2 10.0 263 48 44.5 Alaska
## 3 8.1 294 80 31.0 Arizona
## 4 8.8 190 50 19.5 Arkansas
## 5 9.0 276 91 40.6 California
## 6 7.9 204 78 38.7 Colorado
## 7 3.3 110 77 11.1 Connecticut
## 8 5.9 238 72 15.8 Delaware
## 9 15.4 335 80 31.9 Florida
## 10 17.4 211 60 25.8 Georgia
## # ... with 40 more rows
##
## [[2]][[2]]
## # A tibble: 1 × 1
## x
## <int>
## 1 1
In many applications it is easier and more tangible to separate SQL and R code. Furthermore we oftentimes paste queries together to have something like parameterized statements. There are various solutions for this type of problem but not many for the R language. Hence dbtools provides an own interface to what may be understood as template queries. These templates solve two issues for us:
- Put SQL code where it belongs: a
.sqlfile. - Provide a simple way to pass objects to these queries, using parameters.
The use of these features is simple enough. A template is defined as a character and regions in which parameters are substituted are denoted by two curly braces. Users of Liquid templates may be familiar with this idea. Everything inside these regions is interpreted as R-expression and can contain arbitrary operations. The result of the evaluation should be a character of length one.
templateQuery <- "SELECT {{ sqlName(fieldName) }} FROM `someTable`;"
Query(templateQuery, fieldName = "someField")## Query:
## SELECT `someField` FROM `someTable`;
When such a query lives inside a file we can use a connection object and pass it to Query.
otherTemplateQuery <-
"SELECT `someField` FROM `someTable` WHERE `primaryKey` IN {{ sqlInNums(ids) }};"
writeLines(otherTemplateQuery, tmpFile <- tempfile())
Query(file(tmpFile), ids = 1:10)## Query:
## SELECT `someField` FROM `someTable` WHERE `primaryKey` IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
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