Question

I’m trying to learn R, but I’m stuck on something that seems simple. I know SQL, and the easiest way for me to communicate my question is with that language. Can someone help me with a translation from SQL to R?

I’ve figured out that this:

    SELECT col1, sum(col2) FROM table1 GROUP BY col1

translates into this:

    aggregate(x=table1$col2, by=list(table1$col1), FUN=sum)

And I’ve figured out that this:

    SELECT col1, col2 FROM table1 GROUP BY col1, col2

translates into this:

    unique(table1[,c("col1","col2")])

But what is the translation for this?

    SELECT col1 FROM table1 GROUP BY col1

For some reason, the “unique” function seems to switch to a different return type when working on only one column, so it doesn’t work as I would expect.

-TC

Answer 1

I’m guessing that you are referring to the fact that calling unique on a vector will return a vector, rather than a data frame. Here are a couple of examples that may help:

#Some example data
dat <- data.frame(x = rep(letters[1:2],times = 5),
                  y = rep(letters[3:4],each = 5))
> dat
   x y
1  a c
2  b c
3  a c
4  b c
5  a c
6  b d
7  a d
8  b d
9  a d
10 b d
> unique(dat)
  x y
1 a c
2 b c
6 b d
7 a d
#Unique => vector
> unique(dat$x)
[1] "a" "b"
#Same thing
> unique(dat[,'x'])
[1] "a" "b"
#drop = FALSE preserves the data frame structure
> unique(dat[,'x',drop = FALSE])
  x
1 a
2 b
#Or you can just convert it back (although the default column name is ugly)
> data.frame(unique(dat$x))
  unique.dat.x.
1             a
2             b