Execute a query on a given database connection
The dbSendQuery() method only submits and synchronously executes the
SQL query to the database engine. It does not extract any
records — for that you need to use the dbFetch() method, and
then you must call dbClearResult() when you finish fetching the
records you need. For interactive use, you should almost always prefer
dbGetQuery().
dbSendQuery(conn, statement, ...)
conn |
A DBIConnection object, as returned by
|
statement |
a character string containing SQL. |
... |
Other parameters passed on to methods. |
This method is for SELECT queries only. Some backends may
support data manipulation queries through this method for compatibility
reasons. However, callers are strongly encouraged to use
dbSendStatement() for data manipulation statements.
The query is submitted to the database server and the DBMS executes it,
possibly generating vast amounts of data. Where these data live
is driver-specific: some drivers may choose to leave the output on the server
and transfer them piecemeal to R, others may transfer all the data to the
client – but not necessarily to the memory that R manages. See individual
drivers' dbSendQuery() documentation for details.
dbSendQuery() returns
an S4 object that inherits from DBIResult.
The result set can be used with dbFetch() to extract records.
Once you have finished using a result, make sure to clear it
with dbClearResult().
An error is raised when issuing a query over a closed
or invalid connection,
or if the query is not a non-NA string.
An error is also raised if the syntax of the query is invalid
and all query parameters are given (by passing the params argument)
or the immediate argument is set to TRUE.
The following arguments are not part of the dbSendQuery() generic
(to improve compatibility across backends)
but are part of the DBI specification:
params (default: NULL)
immediate (default: NULL)
They must be provided as named arguments. See the "Specification" sections for details on their usage.
No warnings occur under normal conditions.
When done, the DBIResult object must be cleared with a call to
dbClearResult().
Failure to clear the result set leads to a warning
when the connection is closed.
If the backend supports only one open result set per connection,
issuing a second query invalidates an already open result set
and raises a warning.
The newly opened result set is valid
and must be cleared with dbClearResult().
The param argument allows passing query parameters, see dbBind() for details.
immediate argumentThe immediate argument supports distinguishing between "direct"
and "prepared" APIs offered by many database drivers.
Passing immediate = TRUE leads to immediate execution of the
query or statement, via the "direct" API (if supported by the driver).
The default NULL means that the backend should choose whatever API
makes the most sense for the database, and (if relevant) tries the
other API if the first attempt fails. A successful second attempt
should result in a message that suggests passing the correct
immediate argument.
Examples for possible behaviors:
DBI backend defaults to immediate = TRUE internally
A query without parameters is passed: query is executed
A query with parameters is passed:
params not given: rejected immediately by the database
because of a syntax error in the query, the backend tries
immediate = FALSE (and gives a message)
params given: query is executed using immediate = FALSE
DBI backend defaults to immediate = FALSE internally
A query without parameters is passed:
simple query: query is executed
"special" query (such as setting a config options): fails,
the backend tries immediate = TRUE (and gives a message)
A query with parameters is passed:
params not given: waiting for parameters via dbBind()
params given: query is executed
For updates: dbSendStatement() and dbExecute().
Other DBIConnection generics:
DBIConnection-class,
dbAppendTable(),
dbCreateTable(),
dbDataType(),
dbDisconnect(),
dbExecute(),
dbExistsTable(),
dbGetException(),
dbGetInfo(),
dbGetQuery(),
dbIsReadOnly(),
dbIsValid(),
dbListFields(),
dbListObjects(),
dbListResults(),
dbListTables(),
dbReadTable(),
dbRemoveTable(),
dbSendStatement(),
dbWriteTable()
con <- dbConnect(RSQLite::SQLite(), ":memory:") dbWriteTable(con, "mtcars", mtcars) rs <- dbSendQuery(con, "SELECT * FROM mtcars WHERE cyl = 4") dbFetch(rs) dbClearResult(rs) # Pass one set of values with the param argument: rs <- dbSendQuery( con, "SELECT * FROM mtcars WHERE cyl = ?", params = list(4L) ) dbFetch(rs) dbClearResult(rs) # Pass multiple sets of values with dbBind(): rs <- dbSendQuery(con, "SELECT * FROM mtcars WHERE cyl = ?") dbBind(rs, list(6L)) dbFetch(rs) dbBind(rs, list(8L)) dbFetch(rs) dbClearResult(rs) dbDisconnect(con)
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