问题
So this is VERY strange. RODBC seems to drop the time portion of DateTime SQL columns if the result set is large enough. (The queries are running against an SQL Server 2012 machine, and, yes, when I run them on the SQL Server side they produce identical and proper results, regardless of how many rows are returned.)
For example, the following works perfectly:
myconn <- odbcConnect(dsnName, uid, pwd)
results <- sqlQuery(myconn, "SELECT TOP 100 MyID, MyDateTimeColumn from MyTable ORDER BY MyDateTimeColumn DESC")
close(myconn)
In R, the following works as expected:
> results$MyDateTimeColumn[3]
[1] "2013-07-01 00:01:22 PDT"
which is a valid POSIXct date time. However, when somewhere between 10,000 and 100,000 rows are returned, suddenly the time portion disappears:
myconn <- odbcConnect(dsnName, uid, pwd)
bigResults <- sqlQuery(myconn, "SELECT TOP 100000 MyID, MyDateTimeColumn from MyTable ORDER BY MyDateTimeColumn DESC")
close(myconn)
(same code, simply a larger number of rows returned; NOTE: the exact same row has now lost its time component), R responds:
> bigResults$MyDateTimeColumn[3]
[1] "2013-07-01 PDT"
Note that the time is now missing (this is not a different row; it's the exact same row as previous), as the following shows:
>strptime(results$TriggerTime[3], "%Y-%m-%d %H:%M:%S")
[1] "2013-07-01 00:01:22"
>strptime(bigResults$TriggerTime[3], "%Y-%m-%d %H:%M:%S")
[1] NA
Obviously the work-around is either incremental query-with-append or export-to-CSV-and-import-to-R, but this seems very odd. Anyone ever seen anything like this?
Config: I'm using the latest version of RODBC (1.3-10) and can duplicate the behavior on both an R installation running on Windows x64 and an R installation running on Mac OS X 10.9 (Mavericks).
EDIT #2 Adding output of dput() to compare the objects, per request:
> dput(results[1:10,]$MyDateTimeColumn)
structure(c(1396909903.347, 1396909894.587, 1396909430.903, 1396907996.9, 1396907590.02, 1396906077.887, 1396906071.99, 1396905537.36, 1396905531.413, 1396905231.787), class = c("POSIXct", "POSIXt"), tzone = "")
> dput(bigResults[1:10,]$MyDateTimeColumn)
structure(c(1396854000, 1396854000, 1396854000, 1396854000, 1396854000, 1396854000, 1396854000, 1396854000, 1396854000, 1396854000), class = c("POSIXct", "POSIXt"), tzone = "")
It would appear that the underlying data are actually changing as a result of the number of rows returned by the query, which is downright strange.
回答1:
sqlQuery() has an option called as.is. Setting this to TRUE will pull everything as seen in for example Microsoft SQL Management Studio.
回答2:
This is an older question, but I had similar issues when trying to programmatically read in data from 15 different .accdb. All POSIXct fields were read in correctly for every database except those from the months of March, from which I inferred that it is some sort of daylight-savings time issue.
The solution for me (because I didn't want to have to make multiple queries to a dB and then rbind() everything together) was to alter my function to include the lines
#Get initial tz
current_tz <- Sys.timezone()
Sys.setenv(TZ = "America/Phoenix")
[BODY OF FUNCTION]
Sys.setenv(TZ = curent_tz)
After including these few lines, the day/time fields from the March databases were being read in correctly.
回答3:
I cope with the same problem as well. Even stranger, on a large dataset one column would import both date and time the other column only imported the date.
My advise would be to split the data/time in SQL
myconn <- odbcConnect(dsnName, uid, pwd)
results <- sqlQuery(myconn, "SELECT TOP 100 MyID, format(MyDateTimeColumn,"HH:mm:ss") as MyTimeColumn,format(MyDateTimeColumn,"yyyy-MM-dd") as MyDateColumn from MyTable ORDER BY MyDateTimeColumn DESC")
close(myconn)
Then combine them in R afterwards. Hope it helps.
回答4:
I had the same issue and concluded that it is due to DST:
This fails:
as.POSIXct(c("2015-03-29 01:59:22", "2015-03-29 02:00:04"))
This works:
as.POSIXct(c("2015-03-29 01:59:22", "2015-03-29 02:00:04"), tz="UTC")
I could not find how to force tz="UTC" in default RODBC behavior, however using as.is = TRUE and converting columns myself does the job.
Note: At first I had the impression that it was due to huge result, but in fact it was due to the fact that in huge results I have more chances to cross DST updates.
回答5:
sqlQuery(ch, getSQL(sqlquerypath))
stripped the times off my datetime column.
sqlQuery(ch, getSQL(sqlquerypath), as.is = TRUE)
fixed the issue.
回答6:
It may be a daylight saving issue. If there is a time that doesnt exist in your timezone (because of daylight saving) it may cause something like this.
回答7:
I think this is a case of times being stripped from the dates where the date range includes shifts to/from daylight savings time. If you are selecting periods that don't include daylight savings shifts, the times will be retained (e.g., from 1/1/2007 to 3/1/2007. This could possibly be avoided by changing the system time on your computer to follow a time zone (e.g., Arizona) where there are no daylight savings shifts (sounds bizarre, but it has worked for me).
To overcome this issue, import the DateTimes as characters (using "as.is") and then convert them to POSIXct. You could alternatively use "strptime" which converts to POSIXlt and allows you to specify the format. Here's an example of a SQL query where DateTimes are imported as.is (TRUE), but asociated DataValues are not (FALSE) and then the DateTime is converted to an R date format:
data <- sqlQuery(channel, paste("SELECT LocalDateTime, DataValue FROM DataValues WHERE LocalDateTime >= '1/1/2007 0:00' AND LocalDateTime < '1/1/2008 0:00' ORDER BY LocalDateTime ASC"),as.is=c(TRUE,FALSE))
data$LocalDateTime <- as.POSIXct(totalP$LocalDateTime,tz="MST")
回答8:
Why this happens on large datasets returned from sqlQuery()? I don't know. But was able to workaround it by applying a sql conversion in the sql call:
data <- sqlQuery(channel, "SELECT CONVERT(nvarchar(24), DtTimeField, 21) AS HourDt, * FROM ...
This is your workaround.
来源：https://stackoverflow.com/questions/22899516/rodbc-loses-time-values-of-datetime-when-result-set-is-large