SQL查询加入问题

Question

I have two tables login_activity and event_details . The structure of the tables is as below:

Login_activity
activity_id   |  student_id   |  login_date  | event_id

event_details
event_id   |  event_name   |  event_date

I need to fetch the event_id and the count of student_id from login_activity that match the event_id .

I tried the below query

select event_id,student_id 
from login_activity 
where login_activity.event_id in (select event_id from event_details);

But I am not able to get distinct event_id and count of student_ids for those. I know there has to be a simple solution, but I can't seem to get it. What am I missing?

Answer 1

You need to use a GROUP BY and COUNT

SELECT l.event_id, count(student_ID)
FROM login_activity l
 INNER JOIN event_details e ON l.event_id = e.event_id
GROUP BY l.event_id

You actually don't need the event_details table, unless you want to pull in a specific event_name or event_date

SELECT l.event_id, count(student_ID)
FROM login_activity l
GROUP BY l.event_id

Answer 2

我相信你想在WHERE子句之后使用'GROUP BY student_id'。

Answer 3

Do you need the event_details table at all for your results (ie, do you need to use the name or the date of the event in your query)? If not, you can just use the login_activity table:

    select event_id, count(student_id) as student_count from login_activity group by event_id

If you were wanting the name of the event, it's still much the same, though.

    select e.event_name, count(l.student_id) as student_count
    from event_details e
    left join login_activity l on e.event_id = l.event_id
    group by e.event_name

The advantage of the left join above is that you can get results for events that do not have any login activity yet, if that's applicable. And you can replace event_name with event_id or the date as appropriate.

Answer 4

YOu need either an inner join or a left join to combine the data to get what you need. Then you can use group by and use your event id. To get the count you can use the count command and you can do as a new field for example count(l.student_id) as studentCount.