我不认为您可以在case语句中检查null。相反，您可以使用bincond运算符首先检查null值，然后使用case语句检查其余的“foo”值
假设只有一个字段在几行中有空值

B = FOREACH A GENERATE (foo is null ? 'something' : (case foo    
      when 'S' then '1'
      when 'W' then '2'
      ...
      else '3'
end));

例子

1   1
2   1
3   1
4   2

脚本-为测试生成一些空值并替换为“something”

A = LOAD '/test20.txt' AS (f1:int,f2:int);
B = FOREACH A GENERATE $0;
C = FOREACH A GENERATE $1;
D = JOIN B BY $0 LEFT OUTER,C BY $0;
E = FOREACH D GENERATE $0, ($1 is null ? 'SOMETHING' : (case $1 
                  when 1 then 'One'
                  when 2 then 'Two'
                  when 3 then 'Three'
                  when 4 then 'Four'
                  else 'UNKNOWN'
                  end));

 DUMP E;

输出