Passing characters as argument into db function in R - sql

Question

I am wondering how to pass 2 characters (x,y) in my below R user-defined function, and hope someone could assist on this:
sql.r<-function(x,y){
# Load RODBC package
library(RODBC)
# Create a connection to the database called "con"
con <- odbcConnect("odbccalc", uid=xxx, pwd=xxx, believeNRows=FALSE)
# Check that connection is working (Optional)
odbcGetInfo(con)
# Find out what tables are available (Optional)
Tables <- sqlTables(con, schema="tblData")
# Query the database and put the results into the data frame "dataframe"
dataframe <- sqlQuery(con, "
SELECT lbl,Date, dot
FROM
tblData t
WHERE t.lbl="'',x,"''
AND t.Date <"'',y,"''
ORDER BY t.Date desc")
The syntax problem might lie in the management of quotes.
Working syntax, in case it helps:
sqlQuery(con, "
SELECT lbl,Date, dot
FROM
tblData t
WHERE t.lbl='fruit'
AND t.Date < '2015-06-01'
ORDER BY t.Date desc")
Best,

Answer 1

As others say, you can use paste or paste0 to construct the query. However, the sprintf function can also do the trick. I think this is slightly more easy to read as you avoid mixed single and double quotation marks.
I.e. do something like the following in your function:
query <- sprintf("SELECT lbl, Date, dot
FROM
tblData t
WHERE t.lbl= '%s'
AND t.Date < '%s'
ORDER BY t.Date desc", x, y)
sqlQuery(con, query)

Answer 2

You have to build the query through paste or paste0. Try this:
dataframe <- sqlQuery(con, paste0("
SELECT lbl,Date, dot
FROM
tblData t
WHERE t.lbl='",x,"'
AND t.Date <'",y,"'
ORDER BY t.Date desc"))
The point is that sqlQuery takes two args: a connection and a string. The string is the sql command you want to execute. If the command depends on some inputs, you have to build the string accordingly. paste and sprintf let you do this. You are putting the values of the x and y variables in the string representing the command. This isn't sql specific, but just standard string manipulation.