SQLite 3.27

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create table sequence (__dirpath blob, id int not null);
✓
insert into sequence(__dirpath, id) values
    ("A",2), ("A", 3), ("A", 4), ("A", 6), ("A", 7), ("A", 8), ("A", 9),
    ("A", 10), ("A", 15), ("A", 16), ("A", 17), ("A", 18), ("A", 19), ("A", 20);
✓
insert into sequence(__dirpath, id) values
    ("B", 5), ("B", 6), ("B", 7), ("B", 117), ("B", 14), ("B", 9),
    ("B", 10), ("B", 15), ("B", 16), ("B", 17), ("B", 18), ("B", 19), ("B", 20);
✓
WITH cte AS (
  SELECT __dirpath, id FROM sequence
  UNION ALL
  SELECT DISTINCT __dirpath, '0' FROM sequence 
)
SELECT dir, start, stop 
FROM (
  SELECT m.id + 1 start,
    (SELECT MIN(id) - 1 FROM cte x WHERE x.__dirpath = m.__dirpath AND x.id > m.id) stop, 
     m.__dirpath dir
  FROM cte m LEFT JOIN cte r 
  ON m.__dirpath = r.__dirpath AND m.id = r.id - 1
  WHERE r.id IS NULL
) 
WHERE stop IS NOT NULL 
ORDER BY dir, start, stop;


dir
start
stop


A
1
1

A
5
5

A
11
14

B
1
4

B
8
8

B
11
13

B
21
116