Sql query max, group by - mysql

Question

I am trying to get all students group by class_id, student_id, teacher_id
SO what I mean is this one :
Select id,class_id, student_id,teacher_id, max(active)
FROM student_classes
GROUP BY class_id, student_id, teacher_id
But this is what I get
Actually what I want as a result is:
114 137 1 47 1
108 138 2 49 0
113 197 3 47 1
So basically the problem is at the third row. Instead of having id = 113 I get ID=111.
What should I do in this case? Can you please help me with the query

Answer 1

As mentioned in the comments, MySQL allows something against the SQL standard, letting you include a non-aggregated column (in this case id) in the select list of a query that includes a group by. As far as I know, it will arbitrarily pick one row in each grouping and display the id value from that row.
If you have a specific rule about which id value you want to see, you need to express that in your query.
By the way, your desired output appears to have multiple typos (e.g. 197, which doesn't appear in your data at all).
From your comment (which you should edit into your original question), and your desired output, I think the rule you want for the id column is:
If there are any rows with active=1 in the group, choose the maximum id value from those rows
If all rows in the group have active=0, choose the minimum id value. (You didn't say this specifically; I'm assuming it based on the presence of 108 on the second row of your desired output.)
I think that this query will produce those results. (And also eliminate the non-standard MySQL behavior.)
SELECT
COALESCE(
MAX(CASE WHEN active=1 THEN id ELSE NULL END),
MIN(id)
) AS some_id
class_id, student_id, teacher_id, max(active)
FROM student_classes
GROUP BY class_id, student_id, teacher_id

Answer 2

MySQL versions 5.5, 5.6 works as you coded. But actually it's not correct. With version 5.7 and higher it will throw error. The error will be like "SELECT list is not in GROUP BY clause and contains nonaggregated column 'student_classes.id'..."
Therefore it seems your DB version is old and maybe this code should work as you wanted
select
---------
min(x.id) as id,
---------
x.class_id,
x.student_id,
x.active
from student_classes x
inner join (select
class_id,
student_id,
teacher_id,
---------
max(active) max_active
---------
from student_classes x
group by class_id, student_id, teacher_id
) y
on x.class_id = y.class_id and
x.student_id = y.student_id and
x.teacher_id = y.teacher_id and
x.active = y.max_active
group by x.class_id, x.student_id, x.active
order by id, class_id, student_id
;

Answer 3

You don't want an aggregation actually, but rather pick particular rows. The rule for picking a row is: Per class_id, student_id, teacher_id get the one with the maximum active and in case of a tie the lowest id. This is a ranking of rows.
As of MySQL 8 you can use a window function like ROW_NUMBER to rank rows:
select *
from
(
select
sc.*,
row_number() over (partition by class_id, student_id, teacher_id
order by active desc, id) as rn
from student_classes sc
) with_wanted_id
where rn = 1;
In older versions you could use NOT EXISTS to exclude rows for which a better row exists:
select *
from student_classes sc1
where not exists
(
select null
from student_classes sc2
where sc2.class_id = sc1.class_id
and sc2.student_id = sc1.student_id
and sc2.teacher_id = sc1.teacher_id
and
(
sc2.active > sc1.active
or
(sc2.active = sc1.active and sc2.id < sc1.id)
)
);