选择内部选择计数

Question

I have a table named attendance which has columns roll , class_id , status and att_date .
I have another table named class which has columns class_id and name .
I want to select distinct class_id and count number of roll which has status = 1 where date="some_date" and then connect it to class table using inner join. and again apply where branch ="Computer science"

But I'm facing some problem. This is an example of my table attendance:

roll | class_id | status | att_date
abc  |    1     |   0    | 19-06-2016
cvb  |    2     |   1    | 19-06-2016
nbs  |    1     |   1    | 19-06-2016
lkl  |    3     |   1    | 19-06-2016
ewq  |    3     |   1    | 19-06-2016
dff  |    2     |   1    | 19-06-2016
xyz  |    2     |   1    | 19-06-2016

This is an example of my table class:

id  |  name | branch
1   |  CS4  | Computer Science
2   |  CS5  | Computer Science
3   |  CS6  | Mechanical

and I want something like this :

total number of roll with status 1 | class_id  | name
1                                  |    1      | CS4
3                                  |    2      | CS5
2                                  |    3      | CS6

Can someone explain me ?
How can I approach with the query ?

Answer 1

Use a group by within a group by :

select cnt, count(*) as num_status_1,
       group_concat(a.class_id order by a.class_id) as class_ids,
       group_concat(c.name order by a.class_id) as class_names
from (select class_id, count(*) as cnt 
      from attendance
      where status = 1
      group by class_id
     ) a join
     class c
     on a.class_id = c.class_id
group by cnt;

EDIT:

Note: This aggregates by cnt , and you might not want to do that (your results are ambiguous). This might be sufficient:

select cnt,
       a.class_id, c.nameclass_names
from (select class_id, count(*) as cnt 
      from attendance
      where status = 1
      group by class_id
     ) a join
     class c
     on a.class_id = c.id;

Or even:

select c.*,
       (select count(*) from attendance a where a.status = 1 and a.class_id = c.id)
from class c;

Answer 2

I think this is a more simpler way to do the job:

select a.class_id, b.name, count(a.*) as tot_status_1 
    from attendance a, class b 
    where a.class_id=b.id and a.status=1