library(mongolite)
connection_string = "mongodb+srv://<username>:
<password>@<cluster-name>.mongodb.net/sample_training"
companies_collection = mongo(collection="companies",
                             db="sample_training",
                             url=connection_string)

## Warning: package 'mongolite' was built under R version 4.0.5

companies = companies_collection$aggregate('[
{"$match":{"number_of_employees":{"$gt":0}}},
{"$group":{"_id":"$category_code",
"employee_total": {"$sum":"$number_of_employees"},
"company_total": {"$sum":1}
}},
{"$sort":{"_id": 1}}
]')

df = as.data.frame(companies)
df$mean_employee = df$employee_total/df$company_total

head(df)

##           _id employee_total company_total mean_employee
## 1        <NA>         657498            31   21209.61290
## 2 advertising          21017           307      68.45928
## 3   analytics           1127            22      51.22727
## 4  automotive            525             2     262.50000
## 5     biotech          11959            23     519.95652
## 6   cleantech           7702            29     265.58621