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.

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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.

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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.