SQL joining columns of the same table - sql

Question

I need help on the following SQL query. Let's say we have table_1 with these columns:
number
Customer
list
321
4514
321
2
2
5321
2
5555
If there's a number in the list column, that indicates that is that there is a list of numbers that should refer to that list. Below is a snapshot of how the final table should look. When there's a null value in the customer field it indicates that there is a list, that list number you can find the customers on that list when the number = the list. I need to change the number to make reference to the number the list belongs to.
number
Customer
list
321
4514
321
5321
2
321
5555
2
I've tried with different joins but unsuccessful:
SELECT *
FROM table_1
OUTER JOIN
(SELECT *
FROM TABLE_1
WHERE list IS NOT NULL) AS table_2 ON TABLE_1.list = table_2.list

Answer 1

You say that this is guaranteed to be one level only. So you can have 321->2, but not, say, 321->2->1.
Then, well, let's join and show the joined result:
select
coalesce(ref.number, t1.number) as num,
t1.customer,
ref.list
from table_1 t1
left outer join table_1 ref on ref.list = t1.number
where t1.list is null;

Answer 2

I guess you need to change the data (DML). Here us an example:
DROP TABLE IF EXISTS customer_list
CREATE TABLE customer_list (
number INT,
Customer INT,
list INT
);
INSERT INTO customer_list (number, Customer, list)
VALUES
(321, 4514, NULL),
(321, NULL, 2),
(2, 5321, NULL),
(2, 5555, NULL);
UPDATE A
SET [number] = B.number
,[list] = b.list
FROM customer_list A
INNER JOIN customer_list B
ON A.number = B.list
WHERE B.Customer IS NULL
DELETE FROM customer_list
WHERE Customer IS NULL;
SELECT *
FROM customer_list
If you need only to get the records:
SELECT B.number
,A.customer
,B.List
FROM customer_list A
INNER JOIN customer_list B
ON A.number = B.list
WHERE B.Customer IS NULL