猪 - 如何在加入后引用 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 4 行失败:无效的字段投影.架构中不存在投影字段 [id]

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 "消除运算符的歧义".你想要的是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 relation/bag A 并且有一列叫做 id在其架构中"

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

A::id 表示有来自 A 的 记录，并且有一列名为 id"

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

所以，你会这样做:

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;

<小时>

一个肮脏的选择:

---

A dirty alternative:

只是因为我很懒，当你开始一个接一个地进行多个连接时，消歧变得非常奇怪:使用唯一标识符.

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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