只需按 session_id ，浓缩液 screen_name ，并对每组记录进行计数。如果你不想建造砖瓦罐，你可以使用 collect_list() 而不是 collect() （但我不推荐）。
查询：

add jar /path/to/jars/brickhouse-0.7.1.jar;
create temporary function collect as "brickhouse.udf.collect.CollectUDAF";

select session_id, screen_flow
  , count(*) count
from (
  select session_id
    , concat_ws('->', collect(screen_name)) screen_flow
  from db.table
  group by session_id ) x
group by session_id, screen_flow

输出：

990004916946605-1404157897784   screen1->screen2    1
990004947764274-1435162269418   screen1->screen3    1

输入：-

990004916946605-1404157897784,screen1,1404157898275
990004916946605-1404157897784,screen2,1404157898337
990004947764274-1435162269418,screen1,1435162274044
990004947764274-1435162269418,screen3,1435162274081

下面是Pig式的答案。。

records = LOAD '/user/user/inputfiles/session_id.txt' USING PigStorage(',') AS (session_id:chararray,screen_name:chararray,screnn_launch_time:chararray);

rec_grped = GROUP records BY session_id;

rec_each =  FOREACH rec_grped 
                     {
                       rec_inner_each = FOREACH records GENERATE screen_name;

                       GENERATE group as session_id, REPLACE(BagToString(rec_inner_each),'_','-->') as screen_flow, 1 as cnt;
};

dump rec_each;

输出：-

990004916946605-1404157897784   screen1-->screen2    1
990004947764274-1435162269418   screen1-->screen3    1