How to select other columns of a table when grouping? [duplicate] - mysql

Question

This question already has answers here:
SQL select only rows with max value on a column [duplicate]
(27 answers)
Closed 1 year ago.
Please assume this table:
// mytable
+--------+-------------+---------+
| num | business_id | user_id |
+--------+-------------+---------+
| 3 | 503 | 12 |
| 7 | 33 | 12 |
| 1 | 771 | 13 |
| 2 | 86 | 13 |
| 1 | 772 | 13 |
| 4 | 652 | 14 |
| 4 | 567 | 14 |
+--------+-------------+---------+
I need to group it based on user_id, So, here is my query:
select max(num), user_id from mytable
group by user_id
Here is the result:
// res
+--------+---------+
| num | user_id |
+--------+---------+
| 7 | 12 |
| 2 | 13 |
| 4 | 14 |
+--------+---------+
Now I need to also get the business_id of those rows. Here is the expected result:
// mytable
+--------+-------------+---------+
| num | business_id | user_id |
+--------+-------------+---------+
| 7 | 33 | 12 |
| 2 | 86 | 13 |
| 4 | 567 | 14 | -- This is selected randomly, because of the equality of values
+--------+-------------+---------+
Any idea how can I do that?

Answer 1

You don't group. You filter. One method uses window functions such as row_number():
select t.*
from (select t.*,
row_number() over (partition by user_id order by num desc) as seqnum
from mytable t
) t
where seqnum = 1;
Another method which can have slightly better performance with an index on (user_id, num) is a correlated subquery:
select t.*
from mytable t
where t.num = (select max(t2.num)
from mytable t2
where t2.user_id = t.user_id
);
You should think "group by" when you want to summarize rows. You should think "where" when you want to choose rows with particular characteristics.