When joining on more than two relations, a better option than using multiple left joins is to use a cogroup. Here we use the EmptyBagToNullFields from DataFu to make the code very concise. This code uses the insight that the input1 bag will be empty when there is no match, and flattening this removes the record. If the input2 or input3 bags are …

gistfile1.txt

DEFINE EmptyBagToNullFields datafu.pig.bags.EmptyBagToNullFields();

input1 = LOAD 'input1' using PigStorage(',') AS (val1:INT,val2:INT);
input2 = LOAD 'input2' using PigStorage(',') AS (val1:INT,val2:INT);
input3 = LOAD 'input3' using PigStorage(',') AS (val1:INT,val2:INT);
  
data1 = COGROUP input1 BY val1, input2 BY val1, input3 BY val1;
data2 = FOREACH data1 GENERATE
  FLATTEN(input1),
  FLATTEN(EmptyBagToNullFields(input2)),
  FLATTEN(EmptyBagToNullFields(input3));