MariaDB: Select val_1, max(value_2), group by value_3 - mysql

Question

I have the following columns:
| order_id | client_id | order_timestamp | buyer_id | (all INTs)
It started with the easy-sounding task "Show me the buyer of the last order for each client", so basically
SELECT
client_id,
max(order_timestamp),
buyer_id
FROM table t
GROUP BY client_id;
if GROUP BY would work as one would expect/wish. I know that this is kind of a common problem, but I've never seen this case in particular where you need another value in addition to the one you're grouping by. I guess using the Window functions could help, but we're using MariaDB 10.0, so that's not really an option. I tried different subselect and joins but it always ends with the problem that I can't use the order_id to join, since I have to group by the client_id. It also came to my mind to join using the client_id AND order_timestamp but the combination is not unique in the table, since it's possible to have orders with the exact same (Unix) timestamp for one client or client/buyer combination (so yeah, this would be an edge case, I would need the buyer of the order with the higher order_id, but that's a problem for another day I guess).
If the table was filled like
| order_id | client_id | order_timestamp | buyer_id |
| 1 | 123 | 9876543 | 2 |
| 2 | 123 | 9876654 | 3 |
| 3 | 234 | 9945634 | 2 |
| 4 | 234 | 9735534 | 1 |
I would like to get
| client_id | buyer_id |
------------|----------|
| 123 | 3 |
| 234 | 2 |
Hopefully, somebody can help me, so I can go to sleep in peace tonight.

Answer 1

If your MariaDB version supports window functions you can use ROW_NUMBER():
select t.client_id, t.buyer_id
from (
select *,
row_number() over (partition by client_id order by order_timestamp desc, order_id desc) rn
from tablename
) t
where t.rn = 1
See the demo.
Results:
| client_id | buyer_id |
| --------- | -------- |
| 123 | 3 |
| 234 | 2 |
Without window functions use NOT EXISTS:
select t.client_id, t.buyer_id
from tablename t
where not exists (
select 1 from tablename
where client_id = t.client_id
and (
order_timestamp > t.order_timestamp
or (order_timestamp = t.order_timestamp and order_id > t.order_id)
)
)

Answer 2

If you use max(field), it will pickup the first column of the group condition. In your case first occuring client_id per group which is not what you want.
Try this.
select client_id, order_timestamp, buyer_id from t
where order_timestamp=
(select max(ot) from t as tcopy where tcopy.client_id= t.client_id )
group by client_id;