PostgreSQL Referencing Outer Query in Subquery - sql

Question

I have two Postgres tables (really, more than that, but simplified for the purpose of the question) - one a record of products that have been ordered by customers, and another a historical record of prices per customer and a date they went into effect. Something like this:
'orders' table
customer_id | timestamp | quantity
------------+---------------------+---------
1 | 2015-09-29 16:01:01 | 5
1 | 2015-10-23 14:33:36 | 3
2 | 2015-10-19 09:43:02 | 7
1 | 2015-11-16 15:08:32 | 2
'prices' table
customer_id | effective_time | price
------------+---------------------+-------
1 | 2015-01-01 00:00:00 | 15.00
1 | 2015-10-01 00:00:00 | 12.00
2 | 2015-01-01 00:00:00 | 14.00
I'm trying to create a query that will return every order and its unit price for that customer at the time of the order, like this:
desired result
customer_id | quantity | price
------------+----------+------
1 | 5 | 15.00
1 | 3 | 12.00
2 | 7 | 14.00
1 | 2 | 12.00
This is essentially what I want, but I know that you can't reference an outer query inside an inner query, and I'm having trouble figuring out how to re-factor:
SELECT
o.customer_id,
o.quantity,
p.price
FROM orders o
INNER JOIN (
SELECT price
FROM prices x
WHERE x.customer_id = o.customer_id
AND x.effective_time <= o.timestamp
ORDER BY x.effective_time DESC
LIMIT 1
) p
;
Can anyone suggest the best way to make this work?

Answer 1

Instead of joining an inline view based on the prices table, you can perform a subquery in the SELECT list:
SELECT customer_id, quantity, (
SELECT price
FROM prices p
WHERE
p.customer_id = o.customer_id
AND p.effective_time <= o.timestamp
ORDER BY p.effective_time DESC
LIMIT 1
) AS price
FROM orders o
That does rely on a correlated subquery, which could be bad for performance, but with the way your data are structured I doubt there's a substantially better alternative.

Answer 2

You dont need the subquery, just a plain inner join will do (this assumes there are no duplicate effective_times per customer):
SELECT o.customer_id, o.quantity
,p.price
FROM orders o
JOIN prices p ON p.customer_id = o.customer_id
AND p.effective_time <= o.timestamp
AND NOT EXISTS ( SELECT * FROM prices nx
WHERE nx.customer_id = o.customer_id
AND nx.effective_time <= o.timestamp
AND nx.effective_time > p.effective_time
)
;