RSQLite.Rmd

---
title: "RSQLite"
author: "Hadley Wickham"
date: "`r Sys.Date()`"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{RSQLite}
  %\VignetteEngine{knitr::rmarkdown}
  \usepackage[utf8]{inputenc}
---

```{r, echo = FALSE}
knitr::opts_chunk$set(comment = "#>", collapse = TRUE)
```

RSQLite is the easiest way to use a database from R because the package itself contains [SQLite](https://www.sqlite.org); no external software is needed. This vignette will walk you through the basics of using a SQLite database.

RSQLite is a DBI-compatible interface which means you primarily use functions defined in the DBI package, so you should always start by loading DBI, not RSQLite:

```{r}
library(DBI)
```

## Creating a new database

To create a new SQLite database, you simply supply the filename to `dbConnect()`:

```{r}
mydb <- dbConnect(RSQLite::SQLite(), "my-db.sqlite")
dbDisconnect(mydb)
unlink("my-db.sqlite")
```

If you just need a temporary database, use either `""` (for an on-disk database) or `":memory:"` or `"file::memory:"` (for a in-memory database). This database will be automatically deleted when you disconnect from it.

```{r}
mydb <- dbConnect(RSQLite::SQLite(), "")
dbDisconnect(mydb)
```

## Loading data

You can easily copy an R data frame into a SQLite database with `dbWriteTable()`:

```{r}
mydb <- dbConnect(RSQLite::SQLite(), "")
dbWriteTable(mydb, "mtcars", mtcars)
dbWriteTable(mydb, "iris", iris)
dbListTables(mydb)
```

## Queries

Issue a query with `dbGetQuery()`:

```{r}
dbGetQuery(mydb, 'SELECT * FROM mtcars LIMIT 5')
```

Not all R variable names are valid SQL variable names, so you may need to escape them with `"`:

```{r}
dbGetQuery(mydb, 'SELECT * FROM iris WHERE "Sepal.Length" < 4.6')
```

If you need to insert the value from a user into a query, don't use `paste()`! That makes it easy for a malicious attacker to insert SQL that might damager your database or reveal sensitive information. Instead, use a parameterised query:

```{r}
dbGetQuery(mydb, 'SELECT * FROM iris WHERE "Sepal.Length" < :x', 
  params = list(x = 4.6))
```

This is a little more typing, but much much safer.

## Batched queries

If you run a query and the results don't fit in memory, you can use `dbSendQuery()`, `dbFetch()` and `dbClearResults()` to retrieve the results in batches. By default `dbFetch()` will retrieve all available rows: use `n` to set the maximum number of rows to return.

```{r}
rs <- dbSendQuery(mydb, 'SELECT * FROM mtcars')
while (!dbHasCompleted(rs)) {
  df <- dbFetch(rs, n = 10)
  print(nrow(df))
}
dbClearResult(rs)
```

## Multiple parameterised queries

You can use the same approach to run the same parameterised query with different parameters. Call `dbBind()` to set the parameters:

```{r}
rs <- dbSendQuery(mydb, 'SELECT * FROM iris WHERE "Sepal.Length" < :x')
dbBind(rs, param = list(x = 4.5))
nrow(dbFetch(rs))
dbBind(rs, param = list(x = 4))
nrow(dbFetch(rs))
dbClearResult(rs)
```

You can also pass multiple parameters in one call to `dbBind()`:

```{r}
rs <- dbSendQuery(mydb, 'SELECT * FROM iris WHERE "Sepal.Length" = :x')
dbBind(rs, param = list(x = seq(4, 4.4, by = 0.1)))
nrow(dbFetch(rs))
dbClearResult(rs)
```


## Statements

DBI has new functions `dbSendStatement()` and `dbExecute()`,
which are the counterparts of `dbSendQuery()` and `dbGetQuery()`
for SQL statements that do not return a tabular result,
such as inserting records into a table, updating a table,
or setting engine parameters.
It is good practice, although currently not enforced, to use the new functions
when you don't expect a result.

```{r}
dbExecute(mydb, 'DELETE FROM iris WHERE "Sepal.Length" < 4')
rs <- dbSendStatement(mydb, 'DELETE FROM iris WHERE "Sepal.Length" < :x')
dbBind(rs, param = list(x = 4.5))
dbGetRowsAffected(rs)
dbClearResult(rs)
```