Build SQL query with JOIN and limits - sql

Question

Help me please build PostgreSQL query.
There are 2 tables: products(id, title) and prices(id, product_id, price_type, moment, value)
moment - timestamp, can be in past or future
Assume that price_type has only two option: retail or purchase
But one product may has many retail prices with different moments.
I need select all products with actual retail and purchase prices, where moment less than now.
It's I can done
SELECT
products.id,
products.title_translations AS title,
retail_prices.moment AS ret_moment,
pur_prices.value AS purchase,
retail_prices.value AS retail
FROM products
LEFT OUTER JOIN prices AS pur_prices ON products.id=pur_prices.product_id AND pur_prices.price_type='purchase' AND pur_prices.moment<current_timestamp
LEFT OUTER JOIN prices AS retail_prices ON products.id=retail_prices.product_id AND retail_prices.price_type='retail' AND retail_prices.moment<current_timestamp
ORDER BY products.id;
It works, but returns
product with all prices, but I need only last prices(by moment).

Answer 1

Just use ROW_NUMBER to find what is the last price before current time
with last_prices as (
SELECT
products.id,
products.title_translations AS title,
prices.moment,
prices.value,
prices.price_type,
ROW_NUMBER() OVER (PARTITION BY product_id, price_type
ORDER BY moment DESC) as rn
FROM products
LEFT JOIN prices
ON products.id = prices.product_id
WHERE moment < now()
)
SELECT id, title,
MAX(CASE WHEN price_type = 'retail'
THEN moment
END) as retail_moment,
MAX(CASE WHEN price_type = 'retail'
THEN value
END) as retail_price,
MAX(CASE WHEN price_type = 'purchase'
THEN moment
END) as purchase_moment,
MAX(CASE WHEN price_type = 'purchase'
THEN value
END) as purchase_price
FROM last_prices
WHERE rn = 1
GROUP BY id, title
ORDER BY id

Answer 2

To keep things organized, and straight in my mind, I'd use CTEs to generate two subsets of price data, one for purchase one for retail and assign a row number in ascending sequence with the lowest number having the most recent moment less than the currenttimestmap. And then when we join to these ctes, we only return the lowest number assigned.
With Pur_prices as (SELECT P.*, row_Number() over (partition by product_ID order by moment desc) RN
FROM prices P
WHERE price_Type = 'purchase'
and p.moment < current_timestamp)
, Retail_prices as (SELECT P.*, row_Number() over (partition by product_ID order by moment desc) RN
FROM prices P
WHERE price_Type = 'retail'
and p.moment < current_timestamp)
SELECT
p.id,
p.title_translations AS title,
rp.moment AS ret_moment,
rp.value AS retail,
pp.moment AS Pur_moment,
pp.value AS purchase
FROM products p
LEFT JOIN pur_prices pp
ON p.id=pp.product_id
AND pp.RN = 1 --Only show the most recent price less than current time
LEFT JOIN retail_prices rp
ON p.id=rp.product_id
AND RP.RN = 1 --Only show the most recent price less than current time
ORDER BY p.id;
The end result should be all products regardless if they have a retail or purchase price; but if they do show the retail/purchase pricing for the most recent moment before now. My only concern is this implies all pricing has a moment they start (no null values allowed!)

Answer 3

You may be wanting it be ordered with respect to moment in descending order.
Change
ORDER BY products.id;
to
ORDER BY product.id ASC, moment DESC;