Oracle SQL, how to select first row in a group? - sql

Question

Here is the my SQL fiddle: http://sqlfiddle.com/#!4/75ab7/2
Basically, I have created a table and insert some data into it.
CREATE TABLE subject (
id INT NOT NULL,
seq_num INT NOT NULL,
name VARCHAR(30) NOT NULL
);
INSERT INTO subject
(id, seq_num, name)
VALUES
(1, 1, 'sub_1_1');
INSERT INTO subject
(id, seq_num, name)
VALUES
(2, 1, 'sub_1_2');
INSERT INTO subject
(id, seq_num, name)
VALUES
(3, 2,'sub_2_1');
INSERT INTO subject
(id, seq_num, name)
VALUES
(4, 2, 'sub_2_2');
INSERT INTO subject
(id, seq_num, name)
VALUES
(5, 2, 'sub_2_3');
INSERT INTO subject
(id, seq_num, name)
VALUES
(6, 3, 'sub_3_1');
INSERT INTO subject
(id, seq_num, name)
VALUES
(7, 3, 'sub_3_1');
I run this select statement:
select
LISTAGG(TRIM(id), ',') WITHIN GROUP (ORDER BY 1) AS IDS,
seq_num,
LISTAGG(TRIM(name), ',') WITHIN GROUP (ORDER BY 1) AS NAMES
from
subject
group by
seq_num
order by
seq_num asc
The select statement result:
| ids | seq_num | names |
|-------|---------|-------------------------|
| 1,2 | 1 | sub_1_1,sub_1_2 |
| 3,4,5 | 2 | sub_2_1,sub_2_2,sub_2_3 |
| 6,7 | 3 | sub_3_1,sub_3_1 |
Can I generate something like this?
| ids | seq_num | names |
|-----|---------|---------|
| 1 | 1 | sub_1_1 |
| 3 | 2 | sub_2_1 |
| 6 | 3 | sub_3_1 |
That is only pick the first row in a group.

Answer 1

Use row number:
select
id, seq_num, name
from
(
select id, seq_num, name,
row_number() over (partition by seq_num order by id) rn
from subject
) t
where rn = 1
order by seq_num;
Here is a link to your updated Fiddle:
Demo

Answer 2

You can use keep and first in oracle:
select seq_num,
max(trim(id)) keep (dense_rank first order by trim(id)) as first_id,
max(trim(name)) keep (dense_rank first order by trim(id)) as first_name
from subject
group by seq_num
order by seq_num asc;
Here is the SQL Fiddle.

Answer 3

This should work:
select
min(id) AS IDS,
seq_num,
min(name) AS NAMES
from
subject
group by
seq_num
order by
seq_num asc;
Working Demo
Hope it helps!