RODBC (version 1.2-3)

sqlQuery: Query an ODBC Database

Description

Submit an SQL query to an ODBC database, and retrieve the results.

Usage

sqlQuery(channel, query, errors = TRUE, ..., rows_at_time = 1)
sqlGetResults(channel, as.is = FALSE,  errors = FALSE,
              max = 0, buffsize = 1000,
              nullstring = NA, na.strings = "NA",
              believeNRows = TRUE, dec = getOption("dec"))

Arguments

channel

connection handle as returned by odbcConnect.

query

any valid SQL statement

errors

if TRUE halt and display error, else return -1

...

additional arguments to be passed to sqlGetResults.

rows_at_time

The number of rows to fetch at a time, up to 1024. Not all drivers work correctly with values > 1. See Details.

as.is

which (if any) character columns should be converted, as in read.table? See the details.

max

limit on the number of rows to fetch, with 0 indicating no limit.

buffsize

an initial guess at the number of rows, used if max = 0 and believeNRows == FALSE for the driver.

nullstring

character string to be used when reading SQL_NULL_DATA character items from the database.

na.strings

character string(s) to be mapped to NA when reading character data.

believeNRows

logical. Is the number of rows returned by the ODBC connection believable? This might already be set to false when the channel was opened, and can that setting cannot be overriden.

dec

The character for the decimal place to be assumed when converting character columns to numeric.

Value

• A data frame (possibly with 0 rows) on success. If errors = TRUE, a character vector of error message(s), otherwise error code -1 (general, call odbcGetErrMsg for details) or -2 (no data, which may not be an error as some SQL commands do return no data).

Details

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.

SQL beginners should note that 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 calls first odbcQuery and then sqlGetResults. If finer-grained control is needed, for example over the number of rows fetched, these functions should be called directly or additional arguments passed to sqlQuery.

sqlGetResults is a mid-level function. It should be called after a call to sqlQuery or odbcQuery and used 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 directly: this happens for double, real, integer and smallint columns in the table. All other SQL data types are converted to character strings by the ODBC interface. If the as.is is true for a column, it is returned as character. Otherwise (where detected) date, datetime and timestamp values are converted to "Date" and "POSIXct" values respectively. (Some drivers seem to confuse times with dates, so times may get converted too.) Other types are converted by Rusing type.convert. When character data are to be converted to numeric data, the setting of options("dec") to map the character used up 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_a_time = 1024. However, some drivers may mis-fetch multiple rows, so set this to 1 if the results are incorrect.

See Also

odbcConnect, sqlFetch, sqlSave, sqlTables, odbcQuery

Examples

channel <- odbcConnect("test")
sqlSave(channel, USArrests, rownames = "State", verbose = TRUE)
# options(dec=".") # optional, if DBMS is not locale-aware
## note case of State, Murder, rape are DBMS-dependent.
sqlQuery(channel, paste("select State, Murder from USArrests",
                        "where Rape > 30 order by Murder"))
close(channel)