SQL Question Looking Up Value in Same Table - sql

Question

Trying to use a self join in SQL to look up a value in the table and apply it.
Her's what I got:
+-----------------+-----+--------+-----------+
| Acutal Output | | | |
+-----------------+-----+--------+-----------+
| TRKID | Fac | NewFac | BAG_TRKID |
| 449 | 11 | 11 | 999 |
| 473 | 11 | 11 | 737 |
| 477 | 11 | 11 | 737 |
| 482 | 11 | 11 | 737 |
| 737 | 89 | 89 | |
| Desired Out Put | | | |
| TRKID | Fac | NewFac | BAG_TRKID |
| 449 | 11 | 11 | 999 |
| 473 | 11 | 89 | 737 |
| 477 | 11 | 89 | 737 |
| 482 | 11 | 89 | 737 |
| 737 | 89 | 89 | |
+-----------------+-----+--------+-----------+
Here's the code below. I can't seem to get the table that I want. The Bag TrkID's Facility Num is not becoming the TrkID New Facility Num.
Select
TABLEA.TRKID,
TABLEA.FAC,
NVL(TABLEA.FAC, TABLEB.FAC) as NEWFAC,
TABLEA.BAG_TRKID
FROM
(
Select
HSD. TRKID,
HSD.NLPT as FAC,
SBPD.BAG_TRKID
From
HSD
LEFT JOIN
SBPD
ON
SBPD.BAG_TRKID = HSD. TRKID
Where
HSD.SCANDT BETWEEN ‘Yesterday’ and ‘Today’
) TABLEA
LEFT JOIN
(
Select
HSD. TRKID,
HSD.NLPT as FAC,
SBPD.BAG_TRKID
From
HSD
LEFT JOIN
SBPD
ON
SBPD.BAG_TRKID = HSD. TRKID
Where
HSD.SCANDT BETWEEN ‘Yesterday’ and ‘Today’
) TABLEB
ON
TABLEA.TRKID = TABLEB.BAG_TRKID

Answer 1

Perhaps something like
select a.TrkID, a."Facility Number", a.BAG_TRKID, b.TrkID as "NEW Fac"
from tbl a
left join tbl b on (a.TrkID = b.trk_id_reference)

Answer 2

Given the limited information that you've shared, I was able to achieve the expected output with the following query:
SELECT a.TrkID, a.facility_number, a.bag_trkid, b.facility_number as new_facility_number
FROM test_tbl AS a
LEFT JOIN test_tbl AS b ON a.bag_trkid = b.trkid OR (a.bag_trkid IS NULL AND b.trkid = a.trkid);
You want to get the new_facility_number for a row based on its bag_trkid (which can be achieved by this: LEFT JOIN test_tbl AS b ON a.bag_trkid = b.trkid).
BUT the trick is to account for the cases when the Left Table (which I refer as a) does not have a bag_trkid. In this case, we will keep the new_facility_number to be the same as a.facility_number, joining the tables on the trkid solely: OR (a.bag_trkid IS NULL AND b.trkid = a.trkid)