猪-如何在JOIN之后引用FOREACH中的列? [英] pig - how to reference columns in a FOREACH after a JOIN?

问题描述

A = load 'a.txt' as (id, a1);
B = load 'b.txt as (id, b1);
C = join A by id, B by id;
D = foreach C generate id,a1,b1;
dump D;

第四行失败: Invalid field projection. Projected field [id] does not exist in schema

我尝试更改为A.id，但最后一行失败:ERROR 0: Scalar has more than one row in the output.

I tried to change to A.id but then the last line fails on: ERROR 0: Scalar has more than one row in the output.

推荐答案

您正在寻找的是.您想要的是A::id，而不是A.id.

What you are looking for is the "Disambiguate Operator". What you want is A::id, not A.id.

A.id说:有一个关系/包 A，并且在其架构中有一个名为id的列"

A.id says "there is a relation/bag A and there is a column called id in its schema"

A::id说:有一个来自A的记录，并且有一个名为id的列"

A::id says "there is a record from A and that has a column called id"

因此，您可以这样做:

A = load 'a.txt' as (id, a1);
B = load 'b.txt as (id, b1);
C = join A by id, B by id;
D = foreach C generate A::id,a1,b1;
dump D;

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一个肮脏的选择:

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A dirty alternative:

仅仅因为我很懒，当您开始一个接一个地进行多个连接时，歧义消除变得很奇怪:使用唯一的标识符.

Just because I'm lazy, and disambiguation gets really weird when you start doing multiple joins one after another: use unique identifiers.

A = load 'a.txt' as (ida, a1);
B = load 'b.txt as (idb, b1);
C = join A by ida, B by idb;
D = foreach C generate ida,a1,b1;
dump D;

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