RSQlite - Find values with most occurences in group - r

Question

I'm using RSQlite to import Datasets from an SQlite-Database. There are multiple millions of observations within the Database. Therefor I'd like to do as much as possible of Data selection and aggregation within the Database.
At some point I need to aggregate a character variable. I want to get the value which occures the most ordered by a group. How can I edit the following dplyr-chain so it works also with RSQlite?
library(tidyverse)
library(RSQLite)
# Path to Database
DATABASE="./xxx.db"
# Connect Database
mydb <- dbConnect(RSQLite::SQLite(), DATABASE)
# Load Database
data = tbl(mydb, "BigData")
# Query Database
Summary <- data %>%
filter(year==2020) %>%
group_by(Grouping_variable) %>%
summarize(count=n(),
Item_variable=names(which.max(table(Item_variable))))
Within R that code would do it's job. Querying the database I get an error code Error: near "(": syntax error
Original pipe contains more filters and steps.
Example Database would basically look like:
data.frame(Grouping_variable=c("A","A","B","C","C","C","D","D","D","D"),
year=c(2019,2020,2019,2020,2020,2020,2020,2020,2020,2021),
Item_variable=c("X","Y","Y","X","X","Y","Y","Y","X","X"))
Grouping_variable year Item_Variable
1 A 2019 X
2 A 2020 Y
3 B 2019 Y
4 C 2020 X
5 C 2020 X
6 C 2020 Y
7 D 2020 Y
8 D 2020 Y
9 D 2020 X
10 D 2021 X
Result should look like:
Grouping_variable count Item_variable
<chr> <int> <chr>
1 A 1 Y
2 C 3 X
3 D 3 Y

Answer 1

Assuming that DF is the data frame defined in the question and using SQL we calculate the count of each item within group in the year 2020 giving tmp and then take the row whose count is maximum giving tmp2 - SQLite guarantees that when using group by and max that the other fields come from the row where the maximum was found. Also take the sum of the counts in tmp2 and finally select just the desired columns.
library(sqldf)
sql <- "with tmp as (
select Grouping_variable, count(*) count, Item_variable from DF
where year = 2020
group by Grouping_variable, Item_variable
),
tmp2 as (
select Grouping_variable, max(count), sum(count) count, Item_variable
from tmp
group by Grouping_variable
)
select Grouping_variable, count, Item_variable
from tmp2
"
sqldf(sql)
giving:
Grouping_variable count Item_variable
1 A 1 Y
2 C 3 X
3 D 3 Y
Added
Suppose that DF were a table in your database. This code creates such a database.
library(RSQLite)
m <- dbDriver("SQLite")
con <- dbConnect(m, dbname = "database.sqlite")
dbWriteTable(con, 'DF', DF, row.names = FALSE)
dbDisconnect(con)
then this would run the sql command in the sql string defined above on that database and return the result.
library(RSQLite)
m <- dbDriver("SQLite")
con <- dbConnect(m, dbname = "database.sqlite")
result <- dbGetQuery(con, sql)
dbDisconnect(con)