如何在pgsql中限制组内的项目

Question

我需要 select 按 external_id 和 resolution 分组并按 timestamp 排序的数据，但 仅限于每个组中的前两个 ID。后面的我不知道怎么做。

我试着用简单的查询来做一些事情： SELECT external_id, resolution, string_agg(id::text, ',') FROM some_table GROUP BY external_id, resolution ORDER BY timestamp LIMIT 2 但这还不够 - 该限制适用于整个查询。

来源

id	external_id	resolution	timestamp
1	1	1D	1645941482
2	1	1D	1645941481
3	1	1D	1645941484
4	2	1D	1645941483
5	2	1D	1645941463
6	3	1D	1645941183
7	3	1D	1645941483
8	3	1D	1646941483
8	3	1D	1645741488
10	3	1D	1645941490
11	1	3D	1645941494
12	1	3D	1645941491
13	2	3D	1645941496
14	2	3D	1645941490
15	2	3D	1645941493
16	2	3D	1645941491
17	3	3D	1645941492

预期结果

external_id	resolution	ids
1	1D	1,2
1	3D	11,12
2	1D	4,5
2	3D	13,14
3	1D	6,7
3	3D	17

Answer 1

您可以将 row_number() 与 cte 一起使用到 select N 来自任何组的行数，然后您可以在该结果集上使用 string_agg()。

架构和插入语句：

 create table source(id int, external_id int, resolution varchar(10), timestamp2 timestamp);


 insert into source values(1,   1   ,'1D',  to_timestamp(1645941482));
 insert into source values(2,   1   ,'1D',  to_timestamp(1645941481));
 insert into source values(3,   1   ,'1D',  to_timestamp(1645941484));
 insert into source values(4,   2   ,'1D',  to_timestamp(1645941483));
 insert into source values(5,   2   ,'1D',  to_timestamp(1645941463));
 insert into source values(6,   3   ,'1D',  to_timestamp(1645941183));
 insert into source values(7,   3   ,'1D',  to_timestamp(1645941483));
 insert into source values(8,   3   ,'1D',  to_timestamp(1646941483));
 insert into source values(8,   3   ,'1D',  to_timestamp(1645741488));
 insert into source values(10,  3   ,'1D',  to_timestamp(1645941490));
 insert into source values(11,  1   ,'3D',  to_timestamp(1645941494));
 insert into source values(12,  1   ,'3D',  to_timestamp(1645941491));
 insert into source values(13,  2   ,'3D',  to_timestamp(1645941496));
 insert into source values(14,  2   ,'3D',  to_timestamp(1645941490));
 insert into source values(15,  2   ,'3D',  to_timestamp(1645941493));
 insert into source values(16,  2   ,'3D',  to_timestamp(1645941491));
 insert into source values(17,  3   ,'3D',  to_timestamp(1645941492));

查询：

 with cte as
 (
   select id,external_id, resolution, row_number()over(partition by external_id,resolution order by timestamp2)rn from source
 )
 SELECT external_id, resolution, string_agg(id::text, ',') ids
 FROM cte
 where rn<=2
 GROUP BY external_id, resolution

输出：

external_id	resolution	ids
1	1D	2,1
1	3D	12,11
2	1D	5,4
2	3D	14,16
3	1D	8,6
3	3D	17

db<>fiddle here

如何在pgsql中限制组内的项目

How to limit items inside a group in pgsql

sql

postgresql

greatest-n-per-group

来源

预期结果