R - How to pass date as a argument in the SQL query - sql

Question

I tried this following query:
studentcal3 <- function(x,y)
{
i<- x
print(i)
j <- y
print(j)
demo1 <-sqldf("SELECT a.StudentName, (b.marks/a.marks) as difference from
(select Date, StudentName, marks from studenthistorydemo WHERE Date= i) as a
INNER JOIN (select Date, StudentName, marks from studenthistorydemo where
Date= j) as b on a.marks= i and b.marks= j WHERE a.marks = i and b.marks =
j and a.StudentName=b.StudentName")
}
And then called the function
studentcal3('2014-01-01','2014-01-02')
Error:- Error in rsqlite_send_query(conn#ptr, statement) : no such column: i
How to resolve this?
P.S - I'm new to R

Answer 1

How should the engine know, which parts are external variables? You must use string concatenation (Read about paste()). And be aware, that you have to pass your variables within single quotes and in a culture and language independant format!
studentcal3 <- function(x,y)
{
i<- x
print(i)
j <- y
print(j)
cmd <-paste("SELECT a.StudentName, (b.marks/a.marks) as difference from ",
"(select Date, StudentName, marks from studenthistorydemo WHERE Date='",i,"') as a ",
"INNER JOIN (select Date, StudentName, marks from studenthistorydemo where ",
"Date='",j,"') as b on a.marks='",i,"' and b.marks='",j,
"' WHERE a.marks ='",i,"' and b.marks ='",j,"' and a.StudentName=b.StudentName",sep="")
#demo1 <-sqldf(cmd)
print(cmd)
}
studentcal3('2014-01-01','2014-01-02')
The result
"SELECT a.StudentName, (b.marks/a.marks) as difference from (select
Date, StudentName, marks from studenthistorydemo WHERE
Date='2014-01-01') as a INNER JOIN (select Date, StudentName, marks
from studenthistorydemo where Date='2014-01-02') as b on
a.marks='2014-01-01' and b.marks='2014-01-02' WHERE a.marks
='2014-01-01' and b.marks ='2014-01-02' and a.StudentName=b.StudentName"
And be aware of sql injection! You might build a statment with real parameters to avoid injection attacks.
If this statement really delivers what you expect is another question. You might print it out and paste it into a query window to test it first...