apache-pig - pig -如何在JOIN之后引用FOREACH中的列？

Question

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，但是最后一行失败: ERROR 0: Scalar has more than one row in the output.

Answer 1

您正在寻找的是"Disambiguate Operator"。您想要的是A::id，而不是A.id。
A.id说:“存在一个关系/袋 A，并且在其模式中有一个称为id的列”
A::id说:“存在来自A的记录，并且其中一列名为id”

因此，您将执行以下操作:

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;

一个肮脏的选择:

只是因为我很懒，当您开始一个接一个地进行多个联接时，歧义消除变得很奇怪:使用唯一的标识符。

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;