This tutorial is for you if you need to use a richer set of SQL
features such as data manipulation queries, parameterized queries and
queries performed using SQL’s transaction features. See
vignette("DBI", package = "DBI") for a more basic tutorial
covering connecting to DBMS and executing simple queries.
dbGetQuery() works by calling a number of functions
behind the scenes. If you need more control you can manually build your
own query, retrieve results at your selected rate, and release the
resources involved by calling the same functions.
These functions are:
dbSendQuery() sends the SQL query to the DBMS and
returns a result object. The query is limited to
SELECT statements. If you want to send other statements,
such as INSERT, UPDATE, DELETE,
etc, use dbSendStatement().dbFetch() is called with the result object
returned by dbSendQuery(). It also accepts an argument
specifying the number of rows to be returned, e.g. n = 200.
If you want to fetch all the rows, use n = -1.dbClearResult() is called when you have finished
retrieving data. It releases the resources associated with the
result object.library(DBI)
con <- dbConnect(
RMariaDB::MariaDB(),
host = "db.relational-data.org",
port = 3306,
username = "guest",
password = "relational",
dbname = "sakila"
)
res <- dbSendQuery(con, "SELECT * FROM film WHERE rating = 'G'")
df <- dbFetch(res, n = 3)
dbClearResult(res)
head(df, 3)## film_id title
## 1 2 ACE GOLDFINGER
## 2 4 AFFAIR PREJUDICE
## 3 5 AFRICAN EGG
## description
## 1 A Astounding Epistle of a Database Administrator And a Explorer who must Find a Car in Ancient China
## 2 A Fanciful Documentary of a Frisbee And a Lumberjack who must Chase a Monkey in A Shark Tank
## 3 A Fast-Paced Documentary of a Pastry Chef And a Dentist who must Pursue a Forensic Psychologist in The Gulf of Mexico
## release_year language_id original_language_id rental_duration rental_rate
## 1 2006 1 NA 3 4.99
## 2 2006 1 NA 5 2.99
## 3 2006 1 NA 6 2.99
## length replacement_cost rating special_features
## 1 48 12.99 G Trailers,Deleted Scenes
## 2 117 26.99 G Commentaries,Behind the Scenes
## 3 130 22.99 G Deleted Scenes
## last_update
## 1 2006-02-15 04:03:42
## 2 2006-02-15 04:03:42
## 3 2006-02-15 04:03:42If your dataset is large you may want to fetch a limited number of
rows at a time. As demonstrated below, this can be accomplished by using
a while loop where the function dbHasCompleted() is used to
check for ongoing rows, and dbFetch() is used with the
n = X argument, specifying how many rows to return on each
iteration. Again, we call dbClearResult() at the end to
release resources.
res <- dbSendQuery(con, "SELECT * FROM film")
while (!dbHasCompleted(res)) {
chunk <- dbFetch(res, n = 300)
print(nrow(chunk))
}## [1] 300
## [1] 300
## [1] 300
## [1] 100dbSendQuery() can be used with parameterized SQL
queries. DBI supports two ways to avoid SQL injection attacks from
user-supplied parameters: quoting and parameterized queries.
Quoting of parameter values is performed using the function
dbQuoteLiteral(), which supports many R data types,
including date and time.1
In cases where users may be supplying table or column names to use in
the query for data retrieval, those names or identifiers must also be
escaped. As there may be DBMS-specific rules for escaping these
identifiers, DBI provides the function dbQuoteIdentifier()
to generate a safe string representation.
safe_id <- dbQuoteIdentifier(con, "rating")
safe_param <- dbQuoteLiteral(con, "G")
query <- paste0("SELECT title, ", safe_id, " FROM film WHERE ", safe_id, " = ", safe_param)
query## [1] "SELECT title, `rating` FROM film WHERE `rating` = 'G'"## title rating
## 1 ACE GOLDFINGER G
## 2 AFFAIR PREJUDICE G
## 3 AFRICAN EGG G
## Showing 3 out of 178 rows.The same result can be had by using glue::glue_sql(). It
performs the same safe quoting on any variable or R statement appearing
between braces within the query string.
id <- "rating"
param <- "G"
query <- glue::glue_sql("SELECT title, {`id`} FROM film WHERE {`id`} = {param}", .con = con)
df <- dbGetQuery(con, query)
head(df, 3)## title rating
## 1 ACE GOLDFINGER G
## 2 AFFAIR PREJUDICE G
## 3 AFRICAN EGG GRather than performing the parameter substitution ourselves, we can push it to the DBMS by including placeholders in the query. Different DBMS use different placeholder schemes, DBI passes through the SQL expression unchanged.
MariaDB uses a question mark (?) as placeholder and expects an
unnamed list of parameter values. Other DBMS may use named parameters.
We recommend consulting the documentation for the DBMS you are using. As
an example, a web search for “mariadb parameterized queries” leads to
the documentation for the PREPARE
statement which mentions:
Within the statement, “?” characters can be used as parameter markers to indicate where data values are to be bound to the query later when you execute it.
Currently there is no list of which placeholder scheme a particular DBMS supports.
Placeholders only work for literal values. Other parts of the query,
e.g. table or column identifiers, still need to be quoted with
dbQuoteIdentifier().
For a single set of parameters, the params argument to
dbSendQuery() or dbGetQuery() can be used. It
takes a list and its members are substituted in order for the
placeholders within the query.
params <- list("G")
safe_id <- dbQuoteIdentifier(con, "rating")
query <- paste0("SELECT * FROM film WHERE ", safe_id, " = ?")
query## [1] "SELECT * FROM film WHERE `rating` = ?"## film_id title
## 1 2 ACE GOLDFINGER
## 2 4 AFFAIR PREJUDICE
## 3 5 AFRICAN EGG
## description
## 1 A Astounding Epistle of a Database Administrator And a Explorer who must Find a Car in Ancient China
## 2 A Fanciful Documentary of a Frisbee And a Lumberjack who must Chase a Monkey in A Shark Tank
## 3 A Fast-Paced Documentary of a Pastry Chef And a Dentist who must Pursue a Forensic Psychologist in The Gulf of Mexico
## release_year language_id original_language_id rental_duration rental_rate
## 1 2006 1 NA 3 4.99
## 2 2006 1 NA 5 2.99
## 3 2006 1 NA 6 2.99
## length replacement_cost rating special_features
## 1 48 12.99 G Trailers,Deleted Scenes
## 2 117 26.99 G Commentaries,Behind the Scenes
## 3 130 22.99 G Deleted Scenes
## last_update
## 1 2006-02-15 04:03:42
## 2 2006-02-15 04:03:42
## 3 2006-02-15 04:03:42Below is an example query using multiple placeholders with the MariaDB driver. The placeholders are supplied as a list of values ordered to match the position of the placeholders in the query.
q_params <- list("G", 90)
query <- "SELECT title, rating, length FROM film WHERE rating = ? AND length >= ?"
res <- dbSendQuery(con, query, params = q_params)
dbFetch(res, n = 3)## title rating length
## 1 AFFAIR PREJUDICE G 117
## 2 AFRICAN EGG G 130
## 3 ALAMO VIDEOTAPE G 126When you wish to perform the same query with different sets of
parameter values, dbBind() is used. There are two ways to
use dbBind(). Firstly, it can be used multiple times with
same query.
res <- dbSendQuery(con, "SELECT * FROM film WHERE rating = ?")
dbBind(res, list("G"))
dbFetch(res, n = 3)## film_id title
## 1 2 ACE GOLDFINGER
## 2 4 AFFAIR PREJUDICE
## 3 5 AFRICAN EGG
## description
## 1 A Astounding Epistle of a Database Administrator And a Explorer who must Find a Car in Ancient China
## 2 A Fanciful Documentary of a Frisbee And a Lumberjack who must Chase a Monkey in A Shark Tank
## 3 A Fast-Paced Documentary of a Pastry Chef And a Dentist who must Pursue a Forensic Psychologist in The Gulf of Mexico
## release_year language_id original_language_id rental_duration rental_rate
## 1 2006 1 NA 3 4.99
## 2 2006 1 NA 5 2.99
## 3 2006 1 NA 6 2.99
## length replacement_cost rating special_features
## 1 48 12.99 G Trailers,Deleted Scenes
## 2 117 26.99 G Commentaries,Behind the Scenes
## 3 130 22.99 G Deleted Scenes
## last_update
## 1 2006-02-15 04:03:42
## 2 2006-02-15 04:03:42
## 3 2006-02-15 04:03:42## film_id title
## 1 1 ACADEMY DINOSAUR
## 2 6 AGENT TRUMAN
## 3 12 ALASKA PHANTOM
## description
## 1 A Epic Drama of a Feminist And a Mad Scientist who must Battle a Teacher in The Canadian Rockies
## 2 A Intrepid Panorama of a Robot And a Boy who must Escape a Sumo Wrestler in Ancient China
## 3 A Fanciful Saga of a Hunter And a Pastry Chef who must Vanquish a Boy in Australia
## release_year language_id original_language_id rental_duration rental_rate
## 1 2006 1 NA 6 0.99
## 2 2006 1 NA 3 2.99
## 3 2006 1 NA 6 0.99
## length replacement_cost rating special_features
## 1 86 20.99 PG Deleted Scenes,Behind the Scenes
## 2 169 17.99 PG Deleted Scenes
## 3 136 22.99 PG Commentaries,Deleted Scenes
## last_update
## 1 2006-02-15 04:03:42
## 2 2006-02-15 04:03:42
## 3 2006-02-15 04:03:42Secondly, dbBind() can be used to execute the same
statement with multiple values at once.
res <- dbSendQuery(con, "SELECT * FROM film WHERE rating = ?")
dbBind(res, list(c("G", "PG")))
dbFetch(res, n = 3)## film_id title
## 1 2 ACE GOLDFINGER
## 2 4 AFFAIR PREJUDICE
## 3 5 AFRICAN EGG
## description
## 1 A Astounding Epistle of a Database Administrator And a Explorer who must Find a Car in Ancient China
## 2 A Fanciful Documentary of a Frisbee And a Lumberjack who must Chase a Monkey in A Shark Tank
## 3 A Fast-Paced Documentary of a Pastry Chef And a Dentist who must Pursue a Forensic Psychologist in The Gulf of Mexico
## release_year language_id original_language_id rental_duration rental_rate
## 1 2006 1 NA 3 4.99
## 2 2006 1 NA 5 2.99
## 3 2006 1 NA 6 2.99
## length replacement_cost rating special_features
## 1 48 12.99 G Trailers,Deleted Scenes
## 2 117 26.99 G Commentaries,Behind the Scenes
## 3 130 22.99 G Deleted Scenes
## last_update
## 1 2006-02-15 04:03:42
## 2 2006-02-15 04:03:42
## 3 2006-02-15 04:03:42Use a list of vectors if your query has multiple parameters:
q_params <- list(c("G", "PG"), c(90, 120))
query <- "SELECT title, rating, length FROM film WHERE rating = ? AND length >= ?"
res <- dbSendQuery(con, query, params = q_params)
dbFetch(res, n = 3)## title rating length
## 1 AFFAIR PREJUDICE G 117
## 2 AFRICAN EGG G 130
## 3 ALAMO VIDEOTAPE G 126Always disconnect from the database when done.
For SQL queries that affect the underlying database, such as UPDATE,
DELETE, INSERT INTO, and DROP TABLE, DBI provides two functions.
dbExecute() passes the SQL statement to the DBMS for
execution and returns the number of rows affected.
dbSendStatement() performs in the same manner, but returns
a result object. Call dbGetRowsAffected() with the result
object to get the count of the affected rows. You then need to call
dbClearResult() with the result object afterwards to
release resources.
In actuality, dbExecute() is a convenience function that
calls dbSendStatement(), dbGetRowsAffected(),
and dbClearResult(). You can use these functions if you
need more control over the query process.
The subsequent examples use an in-memory SQL database provided by
RSQLite::SQLite(), because the remote database used in
above examples does not allow writing.
library(DBI)
con <- dbConnect(RSQLite::SQLite(), ":memory:")
dbWriteTable(con, "cars", head(cars, 3))
dbExecute(
con,
"INSERT INTO cars (speed, dist) VALUES (1, 1), (2, 2), (3, 3)"
)## [1] 3rs <- dbSendStatement(
con,
"INSERT INTO cars (speed, dist) VALUES (4, 4), (5, 5), (6, 6)"
)
dbGetRowsAffected(rs)## [1] 3## speed dist
## 1 4 2
## 2 4 10
## 3 7 4
## Showing 3 out of 9 rows.Do not forget to disconnect from the database at the end.
DBI allows you to group multiple queries into a single atomic
transaction. Transactions are initiated with dbBegin() and
either made persistent with dbCommit() or undone with
dbRollback(). The example below updates two tables and
ensures that either both tables are updated, or no changes are persisted
to the database and an error is thrown.
con <- dbConnect(RSQLite::SQLite(), ":memory:")
dbWriteTable(con, "cash", data.frame(amount = 100))
dbWriteTable(con, "account", data.frame(amount = 2000))
withdraw <- function(amount) {
# All operations must be carried out as logical unit:
dbExecute(con, "UPDATE cash SET amount = amount + ?", list(amount))
dbExecute(con, "UPDATE account SET amount = amount - ?", list(amount))
}
withdraw_transacted <- function(amount) {
# Ensure atomicity
dbBegin(con)
# Perform operation
withdraw(amount)
# Persist results
dbCommit(con)
}
withdraw_transacted(300)After withdrawing 300 credits, the cash is increased and the account is decreased by this amount. The transaction ensures that either both operations succeed, or no change occurs.
## amount
## 1 400## amount
## 1 1700We can roll back changes manually if necessary. Do not forget to call
dbRollback() in case of error, otherwise the transaction
remains open indefinitely.
withdraw_if_funds <- function(amount) {
dbBegin(con)
withdraw(amount)
# Rolling back after detecting negative value on account:
if (dbReadTable(con, "account")$amount >= 0) {
dbCommit(con)
TRUE
} else {
message("Insufficient funds")
dbRollback(con)
FALSE
}
}
withdraw_if_funds(5000)## Insufficient funds## [1] FALSE## amount
## 1 400## amount
## 1 1700dbWithTransaction() simplifies using transactions. Pass
it a connection and the code you want to run as a transaction. It will
execute the code and call dbCommit() on success and call
dbRollback() if an error is thrown.
withdraw_safely <- function(amount) {
dbWithTransaction(con, {
withdraw(amount)
if (dbReadTable(con, "account")$amount < 0) {
stop("Error: insufficient funds", call. = FALSE)
}
})
}
withdraw_safely(5000)## Error: Error: insufficient funds## amount
## 1 400## amount
## 1 1700As usual, do not forget to disconnect from the database when done.
That concludes the major features of DBI. For more details on the
library functions covered in this tutorial and the
vignette("DBI", package = "DBI") introductory tutorial see
the DBI specification at vignette("spec", package = "DBI").
If you are after a data manipulation library that works at a higher
level of abstraction, check out dplyr. It is a grammar of data
manipulation that can work with local dataframes and remote databases
and uses DBI under the hood.
An older method, dbQuoteString(), was used
to quote string values only. The dbQuoteLiteral() method
forwards to dbQuoteString() for character vectors. Users do
not need to distinguish between these two cases.↩︎