MySQL不同分类的等量提取问题

这个需要在PostgreSQL中，可以使用窗口查询来满足需求：

digoal=# create table tbl(id int, cate_id int, title text);
CREATE TABLE
digoal=# insert into tbl values (1,1,'a'),(2,1,'b'),(3,2,'c'),(4,2,'d'),(5,2,'e'),(6,3,'f'),(7,3,'g');
INSERT 0 7
digoal=# select * from tbl;
 id | cate_id | title 
----+---------+-------
  1 |       1 | a
  2 |       1 | b
  3 |       2 | c
  4 |       2 | d
  5 |       2 | e
  6 |       3 | f
  7 |       3 | g
(7 rows)
digoal=# select * from (select row_number() over(partition by cate_id) rn,* from tbl) t where rn <=2;
 rn | id | cate_id | title 
----+----+---------+-------
  1 |  1 |       1 | a
  2 |  2 |       1 | b
  1 |  3 |       2 | c
  2 |  4 |       2 | d
  1 |  6 |       3 | f
  2 |  7 |       3 | g
(6 rows)