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.

### 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 be 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.

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

- integer for integer values between -2^31 and 2^31 - 1

- numeric for numbers with a fractional component

- logical for Boolean values (some backends may return an integer)

- character for text

- lists of raw for blobs (with `NULL` entries for SQL NULL values)

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

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

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

- NA for SQL `NULL` values

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. Here, the following rules apply:

- Values are returned as numeric

- 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`, `dbQuoteString`

### 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 0.6-1 Index]