Counting unique / distinct values by group in a data frame

Question

Let\'s say I have the following data frame:

> myvec
    name order_no
1    Amy       12
2   Jack       14
3   Jack       16
4   Dave       11
5    Amy             


        

Answer 1

Here is a solution with sqldf

library("sqldf")

myvec <- read.table(header=TRUE, text=
"   name order_no
1    Amy       12
2   Jack       14
3   Jack       16
4   Dave       11
5    Amy       12
6   Jack       16
7    Tom       19
8  Larry       22
9    Tom       19
10  Dave       11
11  Jack       17
12   Tom       20
13   Amy       23
14  Jack       16")
sqldf("SELECT name,COUNT(distinct(order_no)) as number_of_distinct_orders FROM myvec GROUP BY name")
# > sqldf("SELECT name,COUNT(distinct(order_no)) as number_of_distinct_orders FROM myvec GROUP BY name")
#    name number_of_distinct_orders
# 1   Amy                         2
# 2  Dave                         1
# 3  Jack                         3
# 4 Larry                         1
# 5   Tom                         2