如何按不间断序列对条目进行分组？

Question

CREATE TABLE entries (
  id serial NOT NULL,
  title character varying,
  load_sequence integer
);

和数据

INSERT INTO entries(title, load_sequence) VALUES ('A', 1);
INSERT INTO entries(title, load_sequence) VALUES ('A', 2);
INSERT INTO entries(title, load_sequence) VALUES ('A', 3);

INSERT INTO entries(title, load_sequence) VALUES ('A', 6);

INSERT INTO entries(title, load_sequence) VALUES ('B', 4);
INSERT INTO entries(title, load_sequence) VALUES ('B', 5);

INSERT INTO entries(title, load_sequence) VALUES ('B', 7);
INSERT INTO entries(title, load_sequence) VALUES ('B', 8);

在 PostgreSQL 中有没有一种方法可以编写 SQL 在按 load_sequence 排序后按相同的 title 段对数据进行分组。我的意思是：

=# SELECT id, title, load_sequence FROM entries ORDER BY load_sequence;
 id | title | load_sequence 
----+-------+---------------
  9 | A     |             1
 10 | A     |             2
 11 | A     |             3
 13 | B     |             4
 14 | B     |             5
 12 | A     |             6
 15 | B     |             7
 16 | B     |             8

我想要群组：

=# SELECT title, string_agg(id::text, ',' ORDER BY id) FROM entries ???????????;

所以结果将是：

 title | string_agg  
-------+-------------
 A     | 9,10,11
 B     | 13,14
 A     | 12
 B     | 15,16

Answer 1

您可以使用以下查询：

SELECT title, string_agg(id::text, ',' ORDER BY id)
FROM (
  SELECT id, title, 
         ROW_NUMBER() OVER (ORDER BY load_sequence) -
         ROW_NUMBER() OVER (PARTITION BY title 
                            ORDER BY load_sequence) AS grp
  FROM entries ) AS t
GROUP BY title, grp

计算的 grp 字段用于识别具有连续 load_sequence 值的 title 记录的切片。在 GROUP BY 子句中使用此字段，我们可以实现对 id 值的所需聚合。

Demo here

Answer 2

有一个技巧可以与 sum 一起用作 window 函数运行而不是 lagged window。

这个想法是，当你点击 edge/discontinuity 你 return 1，否则你 return 0。你使用 lag [=21= 检测不连续性] 函数。

SELECT title, string_agg(id::text, ', ') FROM (
  SELECT 
   id, title, load_sequence,
   sum(title_changed) OVER (ORDER BY load_sequence) AS partition_no
  FROM (
    SELECT
      id, title, load_sequence,
      CASE WHEN title = lag(title, 1) OVER (ORDER BY load_sequence) THEN 0 ELSE 1 END AS title_changed FROM entries
  ) x
) y
GROUP BY partition_no, title;

如何按不间断序列对条目进行分组？

How to GROUP entries BY uninterrupted sequence?

sql

postgresql

gaps-and-islands