Postgresql left join

Question

I have two tables cars and usage . I create a record in usage once a month for some of cars. Now I want to get distinct list of cars with their latest usage that I saved.

first of all look at the tables please

cars:

| id | model       | reseller_id |
|----|-------------|-------------|
| 1  | Samand Sall | 324228      |
| 2  | Saba 141    | 92933       |

usages:

| id | car_id | year | month | gas |
|----|--------|------|-------|-----|
| 1  | 2      | 2020 | 2     | 68  |
| 2  | 2      | 2020 | 3     | 94  |
| 3  | 2      | 2020 | 4     | 33  |
| 4  | 2      | 2020 | 5     | 12  |

The problem is here

• I need only the latest usage of year and month

I tried a lot of ways but none of them is good enough. because sometimes this query gets me one ofnot latest records of usages.

SELECT * FROM cars AS c
LEFT JOIN
     (select *
      from usages
     ) u on (c.id = u.car_id)
order by u.gas desc

Answer 1

You can do this with a DISTINCT ON in the derived table:

SELECT * 
FROM cars AS c
  LEFT JOIN (
    select distinct on (u.car_id) *
    from usages u
    order by u.car_id, u.year desc, u.month desc
  ) lu on c.id = lu.car_id
order by u.gas desc;

Answer 2

I think you need window function row_number . Here is the demo .

select
  id,
  model,
  reseller_id
from
(
  select
    c.id,
    model,
    reseller_id,
    row_number() over (partition by u.car_id order by u.id desc) as rn
  from cars c
  left join usages u
  on c.id = u.car_id
) subq
where rn = 1