BigQuery: JOIN on single matching row

Question

I have two tables, one containing orders with a nested line_items structure and another with a pricing history for each product sku code.

Orders Table

order_id	order_date	item_sku	item_quantity	item_subtotal
1	2022-23-07	SKU1	7	12.34
		SKU2	1	9.99
2	2022-12-07	SKU1	1	1.12
		SKU3	5	32.54

Price History Table

item_sku	effective_date	cost
SKU1	2022-20-07	0.78
SKU2	2022-02-03	4.50
SKU1	2022-02-03	0.56
SKU3	2022-02-03	4.32

Desired Output

order_id	order_date	item_sku	item_quantity	item_subtotal	cost
1	2022-23-07	SKU1	7	12.34	0.78
		SKU2	1	9.99	4.50
2	2022-12-07	SKU1	1	1.12	0.56
		SKU3	5	32.54	4.32

I'm trying to get the product cost by finding the cost at the time of the order being placed.

SELECT order_id, order_date,
  ARRAY(
    SELECT AS STRUCT
    item_sku,
    item_quantity,
    item_subtotal,
    cost.product_cost
    FROM UNNEST(line_items) as items
    JOIN `price_history_table` as cost
    ON items.item_sku = cost.sku AND effective_date < order_date 
  ) AS line_items,
  FROM
  `order_data_table`

The above query works but creates a separate line_item array row for each record in the price history table.

How can I match on just the most recent price for that sku. I want to add something like this

ORDER BY effective_date DESC LIMIT 1

But can't work out how to add it.

Answer 1

How can I match on just the most recent price for that sku

You need to add below line into subquery

QUALIFY 1 = ROW_NUMBER() OVER(PARTITION BY item_sku ORDER BY effective_date DESC)    

so, the final query will look like below

SELECT order_id, order_date,
  ARRAY(
    SELECT AS STRUCT
    item_sku,
    item_quantity,
    item_subtotal,
    cost.product_cost
    FROM UNNEST(line_items) as items
    JOIN `price_history_table` as cost
    ON items.item_sku = cost.sku AND effective_date < order_date 
    QUALIFY 1 = ROW_NUMBER() OVER(PARTITION BY item_sku ORDER BY effective_date DESC)
  ) AS line_items,
  FROM
  `order_data_table`           

with output