How to perform a DISTINCT in Pig Latin on a subset of columns?

Question

I would like to perform a DISTINCT operation on a subset of the columns. The documentation says this is possible with a nested foreach:

You cannot us

Answer 1

Group on all the other columns, project just the columns of interest into a bag, and then use FLATTEN to expand them out again:

A_unique =
    FOREACH (GROUP A BY a4) {
        b = A.(a1,a2,a3);
        s = DISTINCT b;
        GENERATE FLATTEN(s), group AS a4;
    };