问题
I have a collection of tuples of the form (t,a,b) that I want to group by b in Pig. Once grouped, I want to filter out b from the tuples in each group and generate a bag of filtered tuples per group.
As an example, assume we have (1,2,1) (2,0,1) (3,4,2) (4,1,2) (5,2,3)
The pig script would produce {(1,2),(2,0)} {(3,4),(4,1)} {(5,2)}
The question is: how do I go about producing this result? I'm used to seeing examples where aggregation operations follow a group by operation. It's less clear to me how to filter the tuples and return them in a bag. Thanks for your assistance!
回答1:
Turns out what I was looking for is the syntax for nested projection in Pig.
If one has tuples of the form (t,a,b) and wants to drop b after the group by, it is done this way.
grouped = GROUP tups BY b;
result = FOREACH grouped GENERATE tup.(t,a);
See the "Nested Projection" section on the PigLatin page. http://wiki.apache.org/pig/PigLatin
来源：https://stackoverflow.com/questions/10808202/projecting-grouped-tuples-in-pig