Actually, i have a method to solve this problem.
But i really want to know, we have to write more statement to do one thing?
First step, we have to create 2 sequence. Let call them "foo" and "foo1".
create sequence foo;
create sequence foo1;
then, you can run below statement, and you will see the result that is we want.
select setval('foo',1);
select setval('foo1',1);
select id, score, class from ( select id, score, ph1.class , sequence1, sequence2, CASE WHEN ph1.cc > 5
THEN ph3.sequence + 4 ELSE ph3.sequence + ph1.cc - 1 END as tail from (select class, count(*) as cc
fromallscore group by class) as ph1 join (select id, class, score, nextval('foo') as
sequence1,currval('foo') as
sequence2 from (select * from allscore order by class, score desc) as t2) as ph2 on (ph1.class = ph2.class) join
(select distinct on (class) class, nextval('foo1'), currval('foo1') as
sequence from ( select id , score , class from allscore order by class, score desc) as t6) as ph3 on
(ph2.class= ph3.class) order by ph1.class , score desc
) as con where sequence2 <= tail;
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