# How to select the first row of each group?

I have a table like this:

ID |  Val   |  Kind
----------------------
1  |  1337  |   2
2  |  1337  |   1
3  |   3    |   4
4  |   3    |   4

I want to make a SELECT that will return just the first row for each Val, ordering by Kind.

Sample output:

ID |  Val   |  Kind
----------------------
2  |  1337  |   1
3  |   3    |   4

How can I build this query?

-
why 3|3|4 and not 4|3|4 - what is the tie-break or do you not care? – Jack Douglas Sep 30 '11 at 18:46
@JackDouglas Actually I have a ORDER BY ID DESC, but that is not relevant for the question. In this example I do not care. – BrunoLM Sep 30 '11 at 21:54

Use a common table expression (CTE) and a windowing/ranking/partitioning function like ROW_NUMBER.

This query will create an in-memory table called ORDERED and add an additional column of rn which is a sequence of numbers from 1 to N. The PARTITION BY indicates it should restart at 1 every time the value of Val changes and we want to order rows by the smallest value of Kind.

WITH ORDERED AS
(
SELECT
ID
,   Val
,   kind
,   ROW_NUMBER() OVER (PARTITION BY Val ORDER BY Kind ASC) AS rn
FROM
mytable
)
SELECT
ID
,   Val
,   Kind
FROM
ORDERED
WHERE
rn = 1;

# Update 2015-05-25

The above approach should work with any RDBMS that has implemented the ROW_NUMBER() function. Oracle has some elegant functionality as expressed in mik's answer below that will generally yield better performance than this answer.

-

bilinkc's solution works fine, but I thought I'd toss mine out as well. It has the same cost, but might be faster (or slower, I haven't tested it). The difference is that it uses the First_Value instead of Row_Number. Since we are only interested in the first value, in my mind it is more straightforward.

SELECT ID, Val, Kind FROM
(
SELECT First_Value(ID) OVER (PARTITION BY Val ORDER BY Kind) First, ID, Val, Kind
FROM mytable
)
WHERE ID = First;

Test Data.

--drop table mytable;
create table mytable (ID Number(5) Primary Key, Val Number(5), Kind Number(5));

insert into mytable values (1,1337,2);
insert into mytable values (2,1337,1);
insert into mytable values (3,3,4);
insert into mytable values (4,3,4);

If you prefer, here is the CTE equivalent.

WITH FirstIDentified AS (
SELECT First_Value(ID) OVER (PARTITION BY Val ORDER BY Kind) First, ID, Val, Kind
FROM mytable
)
SELECT ID, Val, Kind FROM FirstIdentified
WHERE ID = First;
-
+1 but I just thought it worth emphasising that your answer and billinkc's are not logically the same unless id is unique. – Jack Douglas Sep 30 '11 at 19:01
@Jack Douglas - True, I assumed that. – Leigh Riffel Oct 1 '11 at 0:02

This solution also uses keep, but val and kind can also be simply calculated for each group without a CTE:

select min(id) keep(dense_rank first order by kind) id
, val
, min(kind) kind
from mytable
group by val

SQLFiddle here

-

You can use keep to select an id from each group:

select *
from mytable
where id in ( select min(id) keep (dense_rank first order by kind, id)
from mytable
group by val );

ID                     VAL                    KIND
---------------------- ---------------------- ----------------------
2                      1337                   1
3                      3                      4

SQLFiddle Here

-
SELECT MIN(MyTable01.Id) as Id,
MyTable01.Val     as Val,
MyTable01.Kind    as Kind
FROM MyTable MyTable01,
(SELECT Val,MIN(Kind) as Kind
FROM MyTable
GROUP BY Val) MyTableGroup
WHERE MyTable01.Val  = MyTableGroup.Val
AND MyTable01.Kind = MyTableGroup.Kind
GROUP BY MyTable01.Val,MyTable01.Kind
ORDER BY Id;
-
That will be a lot less efficient than the other answers due to the fact that two scans over MyTable are needed. – a_horse_with_no_name Nov 22 '13 at 9:49
That's only true if the optimizer takes the written query literally. More advanced optimizers can see the intent (row per group) and produce a plan with a single table access. – Paul White May 26 '15 at 17:13

## protected by Paul White♦May 26 '15 at 17:14

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