PostgreSQL Order By SubQuery - sql

Question

I have this database structure:
Tables:
Category
id | category
1 | fruit
2 | cars
3 | tables
Product
id | product | category_id
1 | banana | 1
2 | apple | 1
3 | orange | 1
4 | example 1 | 2
5 | example 2 | 3
User_List
id | product_id | user_id | bought_date
1 | 1 | 1 | 2012-06-21 11:00:00
2 | 2 | 1 | 2012-06-21 06:00:00
3 | 4 | 1 | 2012-06-21 08:00:00
4 | 5 | 1 | 2012-06-21 01:00:00
what i want is create a query that "order by bought_date (desc) by category".
In that case the expected result is:
banana
apple
example 1
example 2
My query:
SELECT c.id, u.bought_date
FROM categry as c
left join product p on (c.id=p.category_id)
left join user_list u on (p.id=u.product_id)
WHERE u.user_id=3
ORDER BY u.bought_date DESC NULLS LAST
But this only does a simple sort by bought date...
with this result:
banana
example 1
apple
example 2

Answer 1

I thought of one ordering. You want to order by the earliest or latest date for each category. For that, use window functions.
SELECT c.id, u.bought_date, max(u.bought_date) over (partition by c.id) as category_bd
FROM categry c left join
product p
on (c.id=p.category_id) left join
user_list u
on (p.id=u.product_id)
WHERE u.user_id = 3
ORDER BY category_bd DESC NULLS LAST, u.bought_date DESC NULLS LAST

Answer 2

It sounds as though you just need two columns in your order by clause:
SELECT c.id, u.bought_date
FROM categry as c
left join product p on (c.id=p.category_id)
left join user_list u on (p.id=u.product_id)
WHERE u.user_id=3
ORDER BY category_id, u.bought_date DESC NULLS LAST

Answer 3

Assuming missing information:
bought_date is defined NOT NULL.
There can be multiple rows in user_list per product, and they can have a different bought_date.
Order products by the latest bought_date of their category first,
and by their latest bought_date next.
SELECT p.product
FROM product p
JOIN user_list u ON u.product_id = p.id
GROUP BY 1
ORDER BY max(max(u.bought_date)) OVER (PARTITION BY p.category_id) DESC
, max(u.bought_date) DESC;
You don't need the table categry in the query at all.
The window function to get the latest bought_date per category can go into the ORDER BY clause.
Yes, that's a window function over the aggregate max(u.bought_date).
Obviously you don't want oranges in the result, since nobody "brought" them.
Use JOIN, not LEFT JOIN.