Joining multiple tables and getting MAX value in subquery PostgreSQL - sql

Question

I have 4 Tables in PostgreSQL with the following structure as you can see below:
"Customers"
ID | NAME
101 Max
102 Peter
103 Alex
"orders"
ID | customer_id | CREATED_AT
1 101 2022-05-12
2 101 2022-06-14
3 101 2022-07-9
4 102 2022-02-14
5 102 2022-06-18
6 103 2022-05-22
"orderEntry"
ID | order_id | product_id |
1 3 10
2 3 20
3 3 30
4 5 20
5 5 40
6 6 20
"product"
ID | min_duration
10 P10D
20 P20D
30 P30D
40 P40D
50 P50D
Firstly I need to select "orders" with the max(created_at) date for each customer this is done with the query (it works!):
SELECT c.id as customerId,
o.id as orderId,
o.created_at
FROM Customer c
INNER JOIN Orders o
ON c.id = o.customer_id
INNER JOIN
(
SELECT customer_id, MAX(created_at) Max_Date
FROM Orders
GROUP BY customer_id
) res ON o.customer_id = res.customer_id AND
o.created_at = res.Max_date
the result will look like this:
customer_id | order_id | CREATED_AT
101 3 2022-07-9
102 5 2022-06-18
103 6 2022-05-22
Secondly I need to select for each order_id from "orderEntry" Table, "products" with the max(min_duration) the result should be:
order_id | max(min_duration)
3 P30D
5 P40D
6 P20D
and then join results from 1) and 2) queries by "order_id" and the total result which I'm trying to get should look like this:
customer_name | customer_id | Order_ID | Order_CREATED_AT | Max_Duration
Max 101 3 2022-07-9 P30D
Peter 102 5 2022-06-18 P40D
Alex 103 6 2022-05-22 P20D
I'm struggling to get query for 2) and then join everything with query from 1) to get the result. Any help I would appreciate!

Answer 1

You could make the first query to an CTE and use that to join the rest of the queries.
Like this.
WITH CTE AS ( SELECT c.id as customerId,
o.id as orderId,
o.created_at
FROM Customer c
INNER JOIN Orders o
ON c.id = o.customer_id
INNER JOIN
(
SELECT customer_id, MAX(created_at) Max_Date
FROM Orders
GROUP BY customer_id
) res ON o.customer_id = res.customer_id AND
o.created_at = res.Max_date)
SELECT customerId,orderId,created_at,p.min_duration
FROM CTE
JOIN (SELECT "orderId", MAX("product_id") as product_id FROM "orderEntry" GROUP BY orderId) oe ON CTE.orderId = oe.orderId
JOIN "product" pr ON oe.product_id = pr."ID"