Join on two columns, if null then only join on one - sql

Question

I have the following two tables:
customers:
customer_id
department_id
aaaa
1234
bbbb
3456
status:
department_id
customer_id
status
1234
NULL
silver
3456
bbbb
gold
1234
bbbb
gold
I want to join status on customers, but if if it returns NULL I want to give the customer the department default. My ideal Output for this would be the following:
customer_id
department_id
status
aaaa
1234
silver
bbbb
3456
gold
I have tried to do two left joins, but am getting a memory usage error. Is there another way to do this that is efficient?

Answer 1

You can do:
select c.*, coalesce(s.status, d.status) as status
from customers c
left join status d on d.department_id = c.department_id
and d.customer_id is null
left join status s on s.department_id = c.department_id
and s.customer_id = c.customer_id

Answer 2

This might work:
SELECT *,
(
SELECT TOP 1 status
FROM status s
WHERE s.customer_id = c.customer_id
OR (c.customer_id IS NULL AND s.department_id = c.department_id)
ORDER BY CASE WHEN s.customer_id is NOT NULL THEN 0 ELSE 1 END
) as status
FROM customers c
The kicker is what kind of database you're using. If it's MySql you might want LIMIT 1 instead of TOP 1. For Oracle you'd look at the ROWNUM field.

Answer 3

Assuming that there is always a match at least on the department_id, you need an INNER join and FIRST_VALUE() window function will pick the proper status:
SELECT DISTINCT
c.customer_id,
c.department_id,
FIRST_VALUE(s.status) OVER (
PARTITION BY c.customer_id, c.department_id
ORDER BY CASE
WHEN s.customer_id = c.customer_id THEN 1
WHEN s.customer_id IS NULL THEN 2
ELSE 3
END
) status
FROM customers c INNER JOIN status s
ON s.department_id = c.department_id;
Depending on the database that you use the code may be simplified.
See the demo.