Selecting other columns not in count, group by - postgresql

Question

So I have a table as follows
product_id sender_id timestamp ...other columns...
1 2 1222
1 2 3423
1 2 1231
2 2 890
3 4 234
2 3 234234
I want to get rows where sender_id = 2, but I want to count and group by product_id and sort by timestamp descending. This means I need the following result
product_id sender_id timestamp count ...other columns...
1 2 3423 3
2 2 890 1
I tried the following query:
SELECT product_id, sender_id, timestamp, count(product_id), ...other columns...
FROM table
WHERE sender_id = 2
GROUP BY product_id
But I get the following error Error in query: ERROR: column "table.sender_id" must appear in the GROUP BY clause or be used in an aggregate function
Seems like I cannot SELECT columns that are not in the GROUP BY. Another method which I found online was to join
SELECT product_id, sender_id, timestamp, count, ...other columns...
FROM table
JOIN (
SELECT product_id, COUNT(product_id) AS count
FROM table
GROUP BY (product_id)
) table1 ON table.product_id = table1.product_id
WHERE sender_id = 2
GROUP BY product_id
But doing this simply lists all rows without grouping or counting. My guess is that the ON part simply extends table again.

Answer 1

Try grouping using product_id, sender_id
select product_id, sender_id, count(product_id), max(timestamp) maxtm
from t
where sender_id = 2
group by product_id, sender_id
order by maxtm desc
If you want other columns too:
select t.*, t1.product_count
from t
inner join (
select product_id, sender_id, count(product_id) product_count, max(timestamp) maxtm
from t
where sender_id = 2
group by product_id, sender_id
) t1
on t.product_id = t1.product_id and t.sender_id = t1.sender_id and t.timestamp = t1.maxtm
order by t1.maxtm desc

Answer 2

Just do a workout with your data:
CREATE TABLE products (product_id INTEGER,
sender_id INTEGER,
time_stamp INTEGER)
INSERT INTO products VALUES
(1,2,1222),
(1,2,3423),
(1,2,1231),
(2,2,890),
(3,4,234),
(2,3,234234)
SELECT product_id,sender_id,string_agg(time_stamp::text,','),count(product_id)
FROM products
WHERE sender_id=2
GROUP BY product_id,sender_id
Here you have distinct time_stamp ,so you need to apply some aggregate or just remove that column in select statement.
If you remove time_stamp in select statement then it would be very easy like below :
SELECT product_id,sender_id,count(product_id)
FROM products
WHERE sender_id=2
GROUP BY product_id,sender_id