Self join to create a new column with updated records - sql

Question

I am trying to write a SQL query to get the start date for employees in a store. As seen in the first screenshot, employee number 5041 had the number A0EH but as the number got updated, it updated the start date for the employee as well. This effects the metric of total duration in the store.
I am trying to get to the output below but haven't been able to figure out how to get this view.
This is the code I was trying but I am not getting the correct output.
select
esd.employee_number,
(case when esd.old_employee_number is null then es.employee_number else es.old_employee_number end) as old_employee_number,
esd.entity_id,
esd.original_start_date
from earliest_start_date as esd
left join earliest_start_date as es
on (es.employee_number = esd.old_employee_number)
How do I solve this on SQL?

Answer 1

Redshift reportedly supports recursion via WITH clause. Here's an example:
MariaDB 10.5 has similar support. Test case is here:
Fully working test case (via MariaDB 10.5) (Updated)
Link to Amazon Redshift detail for WITH clause and window functions:
Amazon Redshift - WITH clause
Amazon redshift - Window functions
WITH RECURSIVE cte (employee_number, original_no, entity_id, original_start_date, n) AS (
SELECT employee_number, employee_number, entity_id, original_start_date, 1 FROM earliest_start_date WHERE old_employee_number IS NULL UNION ALL
SELECT new_tbl.employee_number, cte.original_no, cte.entity_id, cte.original_start_date, n+1
FROM earliest_start_date new_tbl
JOIN cte
ON cte.employee_number = new_tbl.old_employee_number
)
, xrows AS (
SELECT *, ROW_NUMBER() OVER (PARTITION BY entity_id ORDER BY n DESC) AS rn
FROM cte
)
SELECT * FROM xrows WHERE rn = 1
;
Result:
+-----------------+-------------+-----------+---------------------+------+----+
| employee_number | original_no | entity_id | original_start_date | n | rn |
+-----------------+-------------+-----------+---------------------+------+----+
| XXXX | XXXX | 88 | 2021-09-02 | 1 | 1 |
| 5041 | A0EH | 96 | 2021-09-05 | 2 | 1 |
+-----------------+-------------+-----------+---------------------+------+----+
2 rows in set
Raw test data:
SELECT * FROM earliest_start_date;
+-----------------+---------------------+-----------+---------------------+
| employee_number | old_employee_number | entity_id | original_start_date |
+-----------------+---------------------+-----------+---------------------+
| 5041 | A0EH | 96 | 2021-09-10 |
| A0EH | NULL | 96 | 2021-09-05 |
| XXXX | NULL | 88 | 2021-09-02 |
+-----------------+---------------------+-----------+---------------------+
Note that the logic makes assumption about uniqueness of the employee_number and, in the current form, can't handle cases where the employee_number is reused by the same employee or used again with a different employee without adjusting prior data. There may not be enough detail in the current structure to handle those cases.