Adding column to sqlite database - r

Question

I am trying to add a vector which I generated in R to a sqlite table as a new column. For this I wanted to use dplyr (I installed the most recent dev. version along with the dbplyr package according to this post here). What I tried:
library(dplyr)
library(DBI)
#creating initial database and table
dbcon <- dbConnect(RSQLite::SQLite(), "cars.db")
dbWriteTable(dbcon, name = "cars", value = cars)
cars_tbl <- dplyr::tbl(dbcon, "cars")
#new values which I want to add as a new column
new_values <- sample(c("A","B","C"), nrow(cars), replace = TRUE)
#attempt to add new values as column to the table in the database
cars_tbl %>% mutate(new_col = new_values) #not working
What is an easy way to achieve this (not necessarily with dplyr)?

Answer 1

Not aware of a way of doing this with dyplr, but you can do it with RSQLite directly. The problem is not actually with RSQLite, but the fact that I don't know how to pass a list to mutate. Note that, in your code, something like this would work:
cars_tbl %>% mutate(new_col = another_column / 3.14)
Anyway, my alternative. I've created a toy cars dataframe.
cars <- data.frame(year=c(1999, 2007, 2009, 2017), model=c("Ford", "Toyota", "Toyota", "BMW"))
I open connection and actually create the table,
dbcon <- dbConnect(RSQLite::SQLite(), "cars.db")
dbWriteTable(dbcon, name = "cars", value = cars)
Add the new column and check,
dbGetQuery(dbcon, "ALTER TABLE cars ADD COLUMN new_col TEXT")
dbGetQuery(dbcon, "SELECT * FROM cars")
year model new_col
1 1999 Ford <NA>
2 2007 Toyota <NA>
3 2009 Toyota <NA>
4 2017 BMW <NA>
And then you can update the new column, but the only tricky thing is that you have to provide a where statement, in this case I use the year.
new_values <- sample(c("A","B","C"), nrow(cars), replace = TRUE)
new_values
[1] "C" "B" "B" "B"
dbGetPreparedQuery(dbcon, "UPDATE cars SET new_col = ? where year=?",
bind.data=data.frame(new_col=new_values,
year=cars$year))
dbGetQuery(dbcon, "SELECT * FROM cars")
year model new_col
1 1999 Ford C
2 2007 Toyota B
3 2009 Toyota B
4 2017 BMW B
As a unique index, you could always use rownames(cars), but you would have to add it as a column in your dataframe and then in your table.
EDIT after suggestion by #krlmlr: indeed much better using dbExecute instead of deprecated dbGetPreparedQuery,
dbExecute(dbcon, "UPDATE cars SET new_col = :new_col where year = :year",
params=data.frame(new_col=new_values,
year=cars$year))
EDIT after comments: I did not think about this a few days ago, but even if it is a SQLite you can use the rowid. I've tested this and it works.
dbExecute(dbcon, "UPDATE cars SET new_col = :new_col where rowid = :id",
params=data.frame(new_col=new_values,
id=rownames(cars)))
Although you have to make sure that the rowid's in the table are the same as your rownames. Anyway you can always get your rowid's like this:
dbGetQuery(dbcon, "SELECT rowid, * FROM cars")
rowid year model new_col
1 1 1999 Ford C
2 2 2007 Toyota B
3 3 2009 Toyota B
4 4 2017 BMW B