Replace subqueries in where statement - sql

Question

I've built a query that intends to find products (products table) with both a 'used' offer and a 'new' offer, and get the lowest price for each. A product can have multiple offers (link_prices table). The offer's condition is determined by the name of the merchant (merchants table): a name without used and occasion is a 'new' offer, a name with used is a 'used' offer.
Here's a sample of the tables (PostgreSQL):
merchants
+----+---------------+
| id | name |
+----+---------------+
| 1 | amazon_used |
| 2 | ebay_location |
| 3 | amazon |
| 4 | target |
| 5 | target_used |
+----+---------------+
link_prices
+----+-------------+------------+-------+
| id | merchant_id | product_id | price |
+----+-------------+------------+-------+
| 1 | 1 | 1 | |
| 2 | 1 | 2 | 20 |
| 3 | 4 | 2 | 30 |
| 4 | 5 | 2 | 5 |
| 5 | 2 | 3 | 10 |
| 6 | 1 | 4 | 80 |
| 7 | 1 | 3 | 100 |
+----+-------------+------------+-------+
In this case, I'm expecting my query to return
+------------+----------------+---------------+
| product_id | min_used_price | min_new_price |
+------------+----------------+---------------+
| 2 | 5 | 30 |
+------------+----------------+---------------+
I've got the following query to work but I feel like I shouldn't need to use subqueries to achieve this. I just can't work my head around it. Any help would be appreciated to optimize this query.
SELECT products.id,
MIN(CASE WHEN merchants.name ILIKE '%used%' THEN link_prices.price END) as min_used_price,
MIN(CASE WHEN merchants.name NOT ILIKE '%used%' THEN link_prices.price END) as min_new_price
FROM products
INNER JOIN link_prices ON link_prices.product_id = products.id
INNER JOIN merchants ON merchants.id = link_prices.merchant_id
WHERE
products.id IN (
SELECT products.id
FROM products
INNER JOIN link_prices ON link_prices.product_id = products.id
INNER JOIN merchants ON merchants.id = link_prices.merchant_id
AND merchants.name ILIKE '%used%'
AND link_prices.price IS NOT NULL
AND link_prices.price <> 0
)
AND products.id IN (
SELECT products.id
FROM products
INNER JOIN link_prices ON link_prices.product_id = products.id
INNER JOIN merchants ON merchants.id = link_prices.merchant_id
AND merchants.name NOT ILIKE '%used%'
AND merchants.name NOT ILIKE '%location%'
AND link_prices.price IS NOT NULL
AND link_prices.price <> 0
)
GROUP BY products.id
Thanks a ton!

Answer 1

Your description makes this sound like conditional aggregation:
select lp.product_id,
min(lp.price) filter (where m.name like '%used') as min_used_price,
min(lp.price) filter (where m.name not like '%used') as min_new_price
from merchants m join
link_prices lp
on lp.merchant_id = m.id
group by lp.product_id;
You sample query is much more complicated and has conditions that are not mentioned in the text of the question. But I think this structure will work for what you want to do.