postgresql获取每组前几条记录

               Table "ytt.t1" 

 Column |         Type          | Modifiers  

--------+-----------------------+----------- 

 i_name | character varying(10) | not null 

 rank   | integer               | not null

t_girl=# select * from t1 order by i_name;

 i_name  | rank  

---------+------ 

 Charlie |   12 

 Charlie |   12 

 Charlie |   13 

 Charlie |   10 

 Charlie |   11 

 Lily       |    6 

 Lily       |    7 

 Lily        |    7 

 Lily       |    6 

 Lily       |    5 

 Lily    |    7 

 Lily    |    4 

 Lucy    |    1 

 Lucy    |    2 

 Lucy    |    2 

 Ytt     |   14 

 Ytt     |   15 

 Ytt     |   14 

 Ytt     |   14 

 Ytt     |   15 

(20 rows)

select *

from (select i_name, rank, row_number() over(partition by i_name) as row from t1) t

where row < =3

SELECT column_name,column_name

FROM table_name;

SELECT * FROM table_name;

SELECT column_name,column_name

FROM table_name

WHERE column_name operator value;