postgresql获取每组前几条记录：
示例
表结构如下：
Table "ytt.t1" Column | Type | Modifiers --------+-----------------------+----------- i_name | character varying(10) | not null rank | integer | not null
模拟了20条数据来做演示：
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)
按照人名分组，每组取3条记录：
select * from (select i_name, rank, row_number() over(partition by i_name) as row from t1) t where row < =3
SELECt 语句用于从数据库中选取数据。
结果被存储在一个结果表中，称为结果集。
SQL SELECT 语法
SELECT column_name,column_name FROM table_name;
与
SELECt * FROM table_name;
SQL WHERe 子句
WHERe 子句用于提取那些满足指定条件的记录。
SQL WHERe 语法
SELECt column_name,column_name FROM table_name WHERe column_name operator value;
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