Am 22.11.15 um 02:38 schrieb John McKown: > On Sat, Nov 21, 2015 at 11:55 AM, Vivek Sutradhara <viveksu...@gmail.com> > wrote: > >> Hi John and Jeff, >> Thanks a lot for your help. I agree that row numbers are not a standard >> feature in SQL. What I am looking for is some kind of a hack. After all, >> the sqlFetch command is able to return a specific number of rows. And the >> sqlFetchMore command is able to take up the baton from that row onwards to >> futher return rows corresponding to the max parameter. >> >> I wonder if it is possible to straight away hop to a certain row number >> (without going through sqlfetch and sqlFetchMore and without loading any >> data into memory) and then return the contents corresponding to a certain >> number of rows. The question is : is there a "catch" for accessing a row >> location, and what could be the "hook" for that? I am interested in the the >> recent updated rows to a table after a certain date. Is it possible to >> identify them in a quick way? Running sql queries on such large tables >> appears to take too long a time. >> >> I understand that there is no provision to do this by available methods. >> But, is it possible to get under the hood and find some hack? >> > > Now you're talking about the internals of Microsoft Access. And you're > _way_ beyond my knowledge. Is there such knowledge? I sure there is. But, > unfortunately, once you get into that depth, you can get into real trouble > when (not if) MS decides to change the internals out from under you without > any warning at all. If you are really needing this, try looking the the > "MDB Tools" software at either https://github.com/brianb/mdbtools or > http://mdbtools.sourceforge.net/ I don't think this does exactly what you > want, but it may give you the information you need to read the MDB file > yourself directly in R code. <shudder/>
The mdb-tools give direct access to the physical mdb files, lying around in a filesystem. A database file xxx.mdb has not to be 'active' within a MS Access 'Server', to read in its contents via mdb-tools. The idea behind is, that one should be able to read the contents of mdb files, even when there is no MS Access you can connect to and/or when no Windows installation is running. In my knowledge, mdb-tools is not available for Windows platforms, only for Unix alikes and Linux, maybe OSX. The R package 'Hmisc' is able to use mdb-tools, if they are also present on that system. Unfortunately, it seems, that mdb-tools also has no direct way to select rows by their number. > > What you would really want is something like the ROWID in SQLite. That is > a "system" maintained column in every table in SQLite. It is a 64-bit > unique number. Basically, it starts at 1 and increments every time you add > a new row. > > What would be "best", IMO, would be if you could alter your Access database > to have a "serial" column which would be your "row number" You could then > get "directly" there by using a SELECT similar to: > > SELECT * FROM table WHERE serial BETWEEN (first-row,last-row) > > > >> >> Jeff, I will take your suggestion and try my luck at the R-sig-db mailing >> list. >> Thanks, >> Vivek >> >> >> ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
