Data.table - left outer join on multiple tables

Question

Suppose you have data like

fruits <- data.table(FruitID=c(1,2,3), Fruit=c(\"Apple\", \"Banana\", \"Strawberry\"))
colors <- data.table(ColorID=c(1,2,3,         


        

Answer 1

I just committed a new feature in data.table, v1.9.5, with which we can join without setting keys (that is, specify the columns to join by directly, without having to use setkey() first):

With that, this is simply:

require(data.table) # v1.9.5+
fruits[tastes, on="FruitID"][colors, on="FruitID"] # no setkey required
#    FruitID      Fruit TasteID  Taste ColorID  Color
# 1:       1      Apple       1 Sweeet       1    Red
# 2:       1      Apple       2   Sour       1    Red
# 3:       1      Apple       1 Sweeet       2 Yellow
# 4:       1      Apple       2   Sour       2 Yellow
# 5:       1      Apple       1 Sweeet       3  Green
# 6:       1      Apple       2   Sour       3  Green
# 7:       2         NA      NA     NA       4 Yellow
# 8:       3 Strawberry       3  Sweet       5    Red

Answer 2

You could use base R's Reduce to left_join (from dplyr) the list of data.table objects at once given that, you are joining the tables with common column names and willing to avoid setting keys multiple times for data.table objects

library(data.table) # <= v1.9.4
library(dplyr) # left_join

Reduce(function(...) left_join(...), list(fruits,colors,tastes))

# Source: local data table [8 x 6]

#  FruitID      Fruit ColorID  Color TasteID  Taste
#1       1      Apple       1    Red       1 Sweeet
#2       1      Apple       1    Red       2   Sour
#3       1      Apple       2 Yellow       1 Sweeet
#4       1      Apple       2 Yellow       2   Sour
#5       1      Apple       3  Green       1 Sweeet
#6       1      Apple       3  Green       2   Sour
#7       2     Banana       4 Yellow      NA     NA
#8       3 Strawberry       5    Red       3  Sweet

Another option with pure data.table approach as @Frank mentioned (Note, this requires the keys to be set to fruitID for all the data.table objects)

library(data.table) # <= v1.9.4
Reduce(function(x,y) y[x, allow.cartesian=TRUE], list(fruits,colors,tastes))