sql problem,challenge - sql

Question

I want to get
id a b c
--------------------
1 1 100 90
6 2 50 100
...from:
id a b c
--------------------
1 1 100 90
2 1 300 50
3 1 200 20
4 2 200 30
5 2 300 70
6 2 50 100
It's the row with the smallest b group by a.
How to do it with sql?
EDIT
I thought it can be achieved by
select * from table group by a having min(b);
which I found later it's wrong.
But is it possible to do it with having statement?
I'm using MySQL

Answer 1

SELECT t1.*
FROM mytable t1
LEFT OUTER JOIN mytable t2
ON (t1.a=t2.a AND t1.b>t2.b)
WHERE t2.a IS NULL;
This works because there should be no matching row t2 with the same a and a lesser b.
update: This solution has the same issue with ties that other folks have identified. However, we can break ties:
SELECT t1.*
FROM mytable t1
LEFT OUTER JOIN mytable t2
ON (t1.a=t2.a AND (t1.b>t2.b OR t1.b=t2.b AND t1.id>t2.id))
WHERE t2.a IS NULL;
Assuming for instance that in the case of a tie, the row with the lower id should be the row we choose.
This doesn't do the trick:
select * from table group by a having min(b);
Because HAVING MIN(b) only tests that the least value in the group is not false (which in MySQL means not zero). The condition in a HAVING clause is for excluding groups from the result, not for choosing the row within the group to return.

Answer 2

In MySQL:
select t1.* from test as t1
inner join
(select t2.a, min(t2.b) as min_b from test as t2 group by t2.a) as subq
on subq.a=t1.a and subq.min_b=t1.b;
Here is the proof:
mysql> create table test (id int unsigned primary key auto_increment, a int unsigned not null, b int unsigned not null, c int unsigned not null) engine=innodb;
Query OK, 0 rows affected (0.55 sec)
mysql> insert into test (a,b,c) values (1,100,90), (1,300,50), (1,200,20), (2,200,30), (2,300,70), (2,50,100);
Query OK, 6 rows affected (0.39 sec)
Records: 6 Duplicates: 0 Warnings: 0
mysql> select * from test;
+----+---+-----+-----+
| id | a | b | c |
+----+---+-----+-----+
| 1 | 1 | 100 | 90 |
| 2 | 1 | 300 | 50 |
| 3 | 1 | 200 | 20 |
| 4 | 2 | 200 | 30 |
| 5 | 2 | 300 | 70 |
| 6 | 2 | 50 | 100 |
+----+---+-----+-----+
6 rows in set (0.00 sec)
mysql> select t1.* from test as t1 inner join (select t2.a, min(t2.b) as min_b from test as t2 group by t2.a) as subq on subq.a=t1.a and subq.min_b=t1.b;
+----+---+-----+-----+
| id | a | b | c |
+----+---+-----+-----+
| 1 | 1 | 100 | 90 |
| 6 | 2 | 50 | 100 |
+----+---+-----+-----+
2 rows in set (0.00 sec)

Answer 3

Use:
SELECT DISTINCT
x.*
FROM TABLE x
JOIN (SELECT t.a,
MIN(t.b) 'min_b'
FROM TABLE T
GROUP BY t.a) y ON y.a = x.a
AND y.min_b = x.b

Answer 4

You're right. select min(b), a from table group by a. If you want the entire row, then you've use analytics function. That depends on database s/w.

Answer 5

It depends on the implementation, but this is usually faster than the self-join method:
SELECT id, a, b, c
FROM
(
SELECT id, a, b, c
, ROW_NUMBER() OVER(PARTITION BY a ORDER BY b ASC) AS [b IN a]
) As SubqueryA
WHERE [b IN a] = 1
Of course it does require that you SQL implementation be fairly up-to-date with the standard.