Submit an SQL query to an ODBC database, and retrieve the results.
sqlQuery(channel, query, errors = TRUE, ..., rows_at_time)sqlGetResults(channel, as.is = FALSE, errors = FALSE,
max = 0, buffsize = 1000,
nullstring = NA_character_, na.strings = "NA",
believeNRows = TRUE, dec = getOption("dec"),
stringsAsFactors = default.stringsAsFactors())
connection handle as returned by odbcConnect.
any valid SQL statement.
logical: if true halt and display error, else return -1.
additional arguments to be passed to sqlGetResults.
The number of rows to fetch at a time, between 1 and 1024. See ‘Details’.
which (if any) columns returned as character should be
converted to another type? Allowed values are as for
read.table. See ‘Details’.
limit on the number of rows to fetch, with 0
indicating no limit.
an initial guess at the number of rows, used if
max = 0 and believeNRows == FALSE.
character string to be used when reading
SQL_NULL_DATA character items from the database.
character vector of strings to be mapped to
NA when reading character data.
logical. Is the number of rows returned by the ODBC connection believable? This might have been set to false when the channel was opened, and if so that setting cannot be overridden.
The character for the decimal place to be assumed when converting character columns to numeric.
logical: should columns returned as character and not
excluded by as.is and not converted to anything else be
converted to factors?
On success, a data frame (possibly with 0 rows) or character string.
On error, if errors = TRUE a character vector of error
message(s), otherwise an invisible integer error code -1
(general, call odbcGetErrMsg for details) or -2
(no data, which may not be an error as some SQL statements do return
no data).
sqlQuery is the workhorse function of RODBC. It sends the SQL
statement query to the server, using connection channel
returned by odbcConnect, and retrieves (some or all of)
the results via sqlGetResults.
The term ‘query’ includes any valid SQL statement including
table creation, alteration, updates etc as well as SELECTs. The
sqlQuery command is a convenience wrapper that first calls
odbcQuery and then sqlGetResults. If
finer-grained control is needed, for example over the number of rows
fetched, additional arguments can be passed to sqlQuery or the
underlying functions called directly.
sqlGetResults is a mid-level function. It is called after a
call to sqlQuery or odbcQuery to retrieve waiting
results into a data frame. Its main use is with max set to
non-zero when it will retrieve the result set in batches with repeated
calls. This is useful for very large result sets which can be
subjected to intermediate processing.
Where possible sqlGetResults transfers data in binary form:
this happens for columns of (ODBC) SQL types double,
real, integer and smallint, and for binary SQL
types (which are transferred as lists of raw vectors, given class
"ODBC_binary"). All other SQL data types are converted to
character strings by the ODBC interface.
This paragraph applies only to SQL data types which are returned by
ODBC as character vectors. If when creating the connection (see
odbcConnect) DBMSencoding was set to a non-empty
value, the character strings are re-encoded. Then if as.is
is true for a column, it is returned as a character vector. Otherwise
(where detected) date, datetime and timestamp
values are converted to the "Date" or "POSIXct" class.
(Some drivers seem to confuse times with dates, so times may get
converted too. Also, some DBMSs (e.g. Oracle's) idea of date
is a date-time.) Remaining cases are converted by R using
type.convert. When character data are to be converted
to numeric data, the setting of options("dec") is used to map
the character used by the ODBC driver in setting decimal points---this
is set to a locale-specific value when RODBC is initialized if
it is not already set.
Using buffsize will yield a marginal increase in speed if set
to no less than the maximum number of rows when believeNRows =
FALSE. (If set too small it can result in unnecessarily high
memory use as the buffers will need to be expanded.)
Modern drivers should work (and work faster, especially if
communicating with a remote machine) with rows_at_time = 100,
the usual default, or more. (However, some drivers may mis-fetch
multiple rows, in which case set rows_at_time = 1 when creating
the connection.) However, if max is specified then this
may fetch too many rows and hence it could be reduced (but then this
setting applies to all subsequent fetches from that result set).
Another circumstance in which you might want to reduce
rows_at_time is if there are large character columns in the
result set: with the default value up to 6Mb of buffer for each such
column could be allocated to store intermediate results.
# NOT RUN {
channel <- odbcConnect("test")
sqlSave(channel, USArrests, rownames = "State", verbose = TRUE)
# options(dec=".") # optional, if DBMS is not locale-aware or set to ASCII
## note case of State, Murder, Rape are DBMS-dependent,
## and some drivers need column and table names double-quoted.
sqlQuery(channel, paste("select State, Murder from USArrests",
"where Rape > 30 order by Murder"))
close(channel)
# }
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