PostgreSQL: Conditionally combine values from two rows - sql

Question

I have inherited a table structure that I have queried to result in the following table:
---------------------------------------------------
| purchase | item | price | category |
---------------------------------------------------
| 1 | widget A | 20.00 | product |
| 1 | widget B | 50.00 | product |
| 2 | widget A | 20.00 | product |
| 3 | widget A | 20.00 | product |
| 3 | promo | 30.00 | product |
| 3 | widget B | 0.00 | bundle |
---------------------------------------------------
And I am trying to extract a specific report that combines the bundled product into the result of the promo item (as they are related), such that it looks like this:
----------------------------------------------------------
| purchase | item | price | category |
----------------------------------------------------------
| 1 | widget A | 20.00 | product |
| 1 | widget B | 50.00 | product |
| 2 | widget A | 20.00 | product |
| 3 | widget A | 20.00 | product |
| 3 | promo - widget B | 30.00 | product |
----------------------------------------------------------
For any item that has a category of 'bundle', the price will be 0.00 and there will be a corresponding promo item as part of that same purchase. For now, I can assume that there is only one bundle product per purchase, but a solution that can concat multiple bundle items to one promo (eg. 'promo - widget B, widget C) would be even better.
Please let me know if I can clarify anything.

Answer 1

This assume only one bundle for purchase. Otherwise you need something to make the pairing.
First you create a field category_type so can group the bundle together.
Then use array_agg to put them together
Finally join the bundles with the normal products.
SQL DEMO
WITH cte as (
SELECT *, CASE WHEN "item" = 'promo' OR "category" = 'bundle'
THEN 'bundle'
ELSE 'product'
END as category_type
FROM purchases
), bundle as (
SELECT purchase,
array_to_string(array_agg(item ), ' - ') as "item",
MAX(price) as price,
MAX(category) as category
FROM cte
WHERE category_type = 'bundle'
GROUP BY purchase
)
SELECT * FROM bundle
UNION ALL
SELECT "purchase", "item", "price", "category"
FROM cte
WHERE category_type <> 'bundle'
ORDER BY "purchase"