R: Fetch records from a previously executed query

dbFetch {DBI} | R Documentation

## Fetch records from a previously executed query

### Description

Fetch the next `n` elements (rows) from the result set and return them as a data.frame.

#### Methods in other packages

- `DBI::dbFetch("DBIResult")`

### Usage

    dbFetch(res, n = -1, ...)

    fetch(res, n = -1, ...)

### Arguments

`res` | An object inheriting from DBIResult, created by `dbSendQuery()`.
`n` | maximum number of records to retrieve per fetch. Use `n = -1` or `n = Inf` to retrieve all pending records. Some implementations may recognize other special values.
`...` | Other arguments passed on to methods.

### Details

`fetch()` is provided for compatibility with older DBI clients - for all new code you are strongly encouraged to use `dbFetch()`. The default implementation for `dbFetch()` calls `fetch()` so that it is compatible with existing code. Modern backends should implement for `dbFetch()` only.

### Value

`dbFetch()` always returns a data.frame with as many rows as records were fetched and as many columns as fields in the result set, even if the result is a single value or has one or zero rows. An attempt to fetch from a closed result set raises an error. If the `n` argument is not an atomic whole number greater or equal to -1 or Inf, an error is raised, but a subsequent call to `dbFetch()` with proper `n` argument succeeds. Calling `dbFetch()` on a result set from a data manipulation query created by `dbSendStatement()` can be fetched and return an empty data frame, with a warning.

### Specification

Fetching multi-row queries with one or more columns by default returns the entire result. Multi-row queries can also be fetched progressively by passing a whole number (integer or numeric) as the `n` argument. A value of Inf for the `n` argument is supported and also returns the full result. If more rows than available are fetched, the result is returned in full without warning. If fewer rows than requested are returned, further fetches will return a data frame with zero rows. If zero rows are fetched, the columns of the data frame are still fully typed. Fetching fewer rows than available is permitted, no warning is issued when clearing the result set.

A column named `row_names` is treated like any other column.

The column types of the returned data frame depend on the data returned:

- integer (or coercible to an integer) for integer values between -2^31 and 2^31 - 1, with NA for SQL `NULL` values

- numeric for numbers with a fractional component, with NA for SQL `NULL` values

- logical for Boolean values (some backends may return an integer); with NA for SQL `NULL` values

- character for text, with NA for SQL `NULL` values

- lists of raw for blobs with NULL entries for SQL NULL values

- coercible using `as.Date()` for dates, with NA for SQL `NULL` values (also applies to the return value of the SQL function `current_date`)

- coercible using `hms::as.hms()` for times, with NA for SQL `NULL` values (also applies to the return value of the SQL function `current_time`)

- coercible using `as.POSIXct()` for timestamps, with NA for SQL `NULL` values (also applies to the return value of the SQL function `current_timestamp`)

If dates and timestamps are supported by the backend, the following R types are used:

- Date for dates (also applies to the return value of the SQL function `current_date`)

- POSIXct for timestamps (also applies to the return value of the SQL function `current_timestamp`)

R has no built-in type with lossless support for the full range of 64-bit or larger integers. If 64-bit integers are returned from a query, the following rules apply:

- Values are returned in a container with support for the full range of valid 64-bit values (such as the `integer64` class of the bit64 package)

- Coercion to numeric always returns a number that is as close as possible to the true value

- Loss of precision when converting to numeric gives a warning

- Conversion to character always returns a lossless decimal representation of the data

### See Also

Close the result set with `dbClearResult()` as soon as you finish retrieving the records you want.

Other DBIResult generics: `DBIResult-class`, `dbBind`, `dbClearResult`, `dbColumnInfo`, `dbGetInfo`, `dbGetRowCount`, `dbGetRowsAffected`, `dbGetStatement`, `dbHasCompleted`, `dbIsValid`, `dbQuoteIdentifier`, `dbQuoteLiteral`, `dbQuoteString`, `dbUnquoteIdentifier`

### Examples

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

    dbWriteTable(con, "mtcars", mtcars)

    # Fetch all results
    rs <- dbSendQuery(con, "SELECT * FROM mtcars WHERE cyl = 4")
    dbFetch(rs)
    dbClearResult(rs)

    # Fetch in chunks
    rs <- dbSendQuery(con, "SELECT * FROM mtcars")
    while (!dbHasCompleted(rs)) {
      chunk <- dbFetch(rs, 10)
      print(nrow(chunk))
    }

    dbClearResult(rs)
    dbDisconnect(con)

[Package DBI version 0.8 Index]