Sort using auxiliary fields, start and end - postgresql

Question

In PostgreSQL, what is the best way to sort records using start and end fields in a generic way, without the need to include in the query the first record (where start_id=3)?
Example table:
+-------+----------+--------+--------+
| FK_ID | START_ID | END_ID | STRING |
+-------+----------+--------+--------+
| 77 | 1 | 9 | E |
| 82 | 5 | 2 | A |
| 77 | 7 | 1 | I |
| 77 | 3 | 7 | W |
| 82 | 9 | 5 | Q |
| 77 | 9 | 5 | X |
| 82 | 2 | 7 | G |
+-------+----------+--------+--------+
Sorted where FK_ID = 77:
+----+---+---+---+
| 77 | 3 | 7 | W |
| 77 | 7 | 1 | I |
| 77 | 1 | 9 | E |
| 77 | 9 | 5 | X |
+----+---+---+---+
Sorted where FK_ID = 82:
+----+---+---+---+
| 82 | 9 | 5 | Q |
| 82 | 5 | 2 | A |
| 82 | 2 | 7 | G |
+----+---+---+---+
Result query sequence:
+-------+----------+
| FK_ID | SEQUENCE |
+-------+----------+
| 82 | QAG |
| 77 | WIEX |
+-------+----------+

Answer 1

I do not think this is the most efficient way but you can try with a recursive CTE
WITH RECURSIVE path AS (
SELECT * FROM myTable AS t1 WHERE NOT EXISTS(
SELECT 1 FROM myTable AS t2 WHERE t1.fk_id = t2.fk_id AND t2.end_id = t1.start_id
) ORDER BY start_id LIMIT 1
UNION ALL
SELECT myTable.* FROM myTable JOIN path ON path.end_id = myTable.start_id
)
SELECT fk_id,array_to_string(array_agg(string)) FROM path GROUP BY fk_id