Compute the number of distinct values in col2 for each distinct value in col1 in R

Question

I have a dataframe like this:

df <- data.frame(
          SchoolID=c(\"A\",\"A\",\"B\",\"B\",\"C\",\"D\"),
          Country=c(\"XX\",\"XX\",\"XX\",\"YY\"         


        

Answer 1

One approach, which does not rely on third-party libraries:

> as.data.frame(rowSums(table(df[!duplicated(df), ]), na.rm=T))
  rowSums(table(df[!duplicated(df), ]), na.rm = T)
A                                                1
B                                                2
C                                                1
D                                                1

Answer 2

aggregate(Country ~ SchoolID, df, function(x) length(unique(x)))

Or

tapply(df$Country, df$SchoolID, function(x) length(unique(x)))

Or

library(data.table) 
setDT(df)[, .(NumberOfCountry = length(unique(Country))), by = SchoolID]

Or with v >1.9.5

setDT(df)[, .(NumberOfCountry = uniqueN(Country)), by = SchoolID]

Or

library(dplyr)
df %>% 
  group_by(SchoolID) %>% 
  summarise(NumberOfCountry = n_distinct(Country))

Answer 3

try this..

select School,count(Country)
from(
select distinct School,Country
from tbl_stacko) temp
group by School