How to get columns which are not in GROUP BY?

I have a Postgresql database where I have these two tables.

shipping_method

id | name     | abbrev
---+----------+-------
1  | Standard | ST
2  | Express  | EX

shipping_details:

id	shipping_method_id	estimated_time_min	estimated_time_max	price
2	1	02:00:00	04:00:00	230
3	2	00:03:00	01:00:00	500
4	1	02:00:00	04:00:00	1230
5	1	02:00:00	04:00:00	850
6	2	01:00:00	02:00:00	1785

My goal is to fetch the most expensive shipping details per shipping method (for a specific product [not in OP]).

So far, I wrote this query:

SELECT 
    sm.id, sm.name, MAX(sd.price) AS max_price
FROM 
    shipping_details AS sd 
LEFT JOIN 
    shipping_method AS sm ON sm.id = sd.shipping_method_id
GROUP BY 
    sm.id

which returns:

id | name     | max_price
---+----------+---------
2  | Express  | 1785
1  | Standard | 1230

Through that query, I am not able to get the shipping_details columns without putting them in GROUP BY clause. I mainly need shipping details for each specific shipping method that has the higher price.

How can I achieve that?

Answer

Use DISTINCT ON:

SELECT DISTINCT ON (sm.id) sm.id, sm.name, sd.price AS max_price
FROM shipping_details AS sd 
LEFT JOIN shipping_method AS sm
    ON sm.id = sd.shipping_method_id
ORDER BY sm.id, sd.price DESC;

The above logic will return the shipping method having the max price.