[R] Reference a variable inside a string and another for object assingments

[Gabor Grothendieck]

On Thu, Aug 16, 2012 at 3:30 PM, Kenneth Rose <kennethrose82 at gmail.com> wrote:
> Hi R community
>
> I copied a bit of my R code that gets some data from a database. You
> won't be able to run the code, but I am a beginner so you will
> probably understand what going on.
>
> I would like to make a variable I can refer to inside the sqlQuery.
> Instead of writing the start date and time (ex SP.lokaldatotid >=
> '2005-01-01 00:00:00') inside the query I would like to define it in
> the beginning of the code, so I don't have to fiddle with a lot of
> dates each time I wan't to change it. I would like to do this for a
> few of the variables and maybe even make a list/array I can loop
> through, so I don't have to write the same code multiple times (for
> SYS and DK1).
>
> I have searched for a solution for two days now, but I am not sure
> what it's called and are probably writing the wrong queries :-)
>
>
> Thank you for your help!
>
> Kenneth
>
> My code:
>
> library(xts)
> library(RODBC)
>
>
> #Define channnel (i configured my own SYSTEM-DNS, via ODBC)
> ch <- odbcConnect("DI2")
>
> #####################################################################
> ############################## GET DATA    ##########################
> #####################################################################
>
> ############################## SYSTEM spot ##########################
> # Hent data fra SQL Server
> sys <- sqlQuery (ch, paste("SELECT  SP.lokaldatotid, SP.pris FROM
> DataIndsamling2.dbo.SpotPriser SP",
>                            "WHERE (SP.omraade_id= 0 AND
> SP.lokaldatotid >= '2005-01-01 00:00:00')"))
> #Definer dato og tid kolonne
> sys$lokaldatotid <- as.POSIXct(sys$lokaldatotid)
>
> #Make a XTS object
> sys_xts <- xts(sys[,-1], order.by=sys[,1])
>
> # Recalculate data from hours to daily, monthly and yearly averages
> sys_xts_daily <- apply.daily(sys_xts, FUN=mean)
> sys_xts_monthly <- apply.monthly(sys_xts, FUN=mean)
> sys_xts_yearly <- apply.yearly(sys_xts, FUN=mean)
>
>
> ############################## DK1 spot #############################
> # Hent data fra SQL Server
> dk1 <- sqlQuery (ch, paste("SELECT  SP.lokaldatotid, SP.pris FROM
> DataIndsamling2.dbo.SpotPriser SP",
>                            "WHERE (SP.omraade_id= 5 AND
> SP.lokaldatotid >= '2005-01-01 00:00:00')"))

You can use paste as shown in the example in ?sqlGetResults or
fn$ in the gsubfn package can do quasi-perl-like string interpolation.
With fn you just preface any command with fn$ and then its
arguments are subject to string interpolation as explained further
in ?fn and http://gsubfn.googlecode.com. e.g.

library(gsubfn)

id <- 5
date <- '2005-01-01 00:00:00'

dk1 <- fn$sqlQuery (ch, "SELECT  SP.lokaldatotid, SP.pris FROM
	DataIndsamling2.dbo.SpotPriser SP
	WHERE (SP.omraade_id = $id AND
	SP.lokaldatotid >= '$date' )" )

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