How to GROUP entries BY uninterrupted sequence? - sql

Question

CREATE TABLE entries (
id serial NOT NULL,
title character varying,
load_sequence integer
);
and data
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);
Is there a way in PostgreSQL to write SQL that groups data by same title segments after ordering them by load_sequence.
I mean:
=# 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
AND I want groups:
=# SELECT title, string_agg(id::text, ',' ORDER BY id) FROM entries ???????????;
so result would be:
title | string_agg
-------+-------------
A | 9,10,11
B | 13,14
A | 12
B | 15,16

Answer 1

You can use the following query:
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
Calculated grp field serves to identify slices of title records having consecutive load_sequence values. Using this field in the GROUP BY clause we can achieve the required aggregation over id values.
Demo here

Answer 2

There's a trick you can use with sum as a window function running over a lagged window for this.
The idea is that when you hit an edge/discontinuity you return 1, otherwise you return 0. You detect the discontinuities using the lag window function.
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;