dbColumnInfo | R Documentation |
Produces a data.frame that describes the output of a query. The data.frame should have as many rows as there are output fields in the result set, and each column in the data.frame describes an aspect of the result set field (field name, type, etc.)
\Sexpr[results=rd,stage=render]{DBI:::methods_as_rd("dbColumnInfo")}dbColumnInfo(res, ...)
res |
An object inheriting from DBIResult. |
... |
Other arguments passed on to methods. |
dbColumnInfo()
returns a data frame
with at least two columns "name"
and "type"
(in that order)
(and optional columns that start with a dot).
The "name"
and "type"
columns contain the names and types
of the R columns of the data frame that is returned from dbFetch()
.
The "type"
column is of type character
and only for information.
Do not compute on the "type"
column, instead use dbFetch(res, n = 0)
to create a zero-row data frame initialized with the correct data types.
This section gives a complete overview over the flow for the execution of queries that return tabular data as data frames.
Most of this flow, except repeated calling of dbBind()
or dbBindArrow()
,
is implemented by dbGetQuery()
, which should be sufficient
unless you want to access the results in a paged way
or you have a parameterized query that you want to reuse.
This flow requires an active connection established by dbConnect()
.
See also vignette("dbi-advanced")
for a walkthrough.
Use dbSendQuery()
to create a result set object of class
DBIResult.
Optionally, bind query parameters with dbBind()
or dbBindArrow()
.
This is required only if the query contains placeholders
such as ?
or $1
, depending on the database backend.
Optionally, use dbColumnInfo()
to retrieve the structure of the result set
without retrieving actual data.
Use dbFetch()
to get the entire result set, a page of results,
or the remaining rows.
Fetching zero rows is also possible to retrieve the structure of the result set
as a data frame.
This step can be called multiple times.
Only forward paging is supported, you need to cache previous pages
if you need to navigate backwards.
Use dbHasCompleted()
to tell when you're done.
This method returns TRUE
if no more rows are available for fetching.
Repeat the last four steps as necessary.
Use dbClearResult()
to clean up the result set object.
This step is mandatory even if no rows have been fetched
or if an error has occurred during the processing.
It is good practice to use on.exit()
or withr::defer()
to ensure that this step is always executed.
An attempt to query columns for a closed result set raises an error.
A column named row_names
is treated like any other column.
The column names are always consistent
with the data returned by dbFetch()
.
If the query returns unnamed columns,
non-empty and non-NA
names are assigned.
Column names that correspond to SQL or R keywords are left unchanged.
Other DBIResult generics:
DBIResult-class
,
dbBind()
,
dbClearResult()
,
dbFetch()
,
dbGetInfo()
,
dbGetRowCount()
,
dbGetRowsAffected()
,
dbGetStatement()
,
dbHasCompleted()
,
dbIsReadOnly()
,
dbIsValid()
,
dbQuoteLiteral()
,
dbQuoteString()
con <- dbConnect(RSQLite::SQLite(), ":memory:")
rs <- dbSendQuery(con, "SELECT 1 AS a, 2 AS b")
dbColumnInfo(rs)
dbFetch(rs)
dbClearResult(rs)
dbDisconnect(con)
Add the following code to your website.
For more information on customizing the embed code, read Embedding Snippets.