How to limit a query by column value - mysql

Question

Following query...
SELECT event_id, user_id FROM EventUser WHERE user_id IN (1, 2)
...gives me the following result:
+----------+---------+
| event_id | user_id |
+----------+---------+
| 3 | 1 |
| 2 | 1 |
| 1 | 1 |
| 5 | 1 |
| 4 | 1 |
| 6 | 1 |
| 4 | 2 |
| 2 | 2 |
| 1 | 2 |
| 5 | 2 |
+----------+---------+
Now, I want to modify the above query so that I only get for example two rows for each user_id, eg:
+----------+---------+
| event_id | user_id |
+----------+---------+
| 3 | 1 |
| 2 | 1 |
| 4 | 2 |
| 5 | 2 |
+----------+---------+
I am thinking about something like this, which of course does not work:
SELECT event_id, user_id FROM EventUser WHERE user_id IN (1, 2) LIMIT 2 by user_id
Ideally, this should work with offsets as well because I want to use it for paginations.
For performance reasons it is essential to use the WHERE user_id IN (1, 2) part of the query.

Answer 1

One method -- assuming you have at least two rows for each user -- would be:
(select min(event_id) as event_id, user_id
from t
where user in (1, 2)
group by user_id
) union all
(select max(event_id) as event_id, user_id
from t
where user in (1, 2)
group by user_id
);
Admittedly, this is not a "general" solution, but it might be the simplest solution for what you want.
If you want the two biggest or smallest, then an alternative also works:
select t.*
from t
where t.user_id in (1, 2) and
t.event_id >= (select t2.event_id
from t t2
where t2.user_id = t.user_id
order by t2.event_id desc
limit 1, 1
);

Answer 2

Here is a dynamic example for such problems, Please note that this example is working in SQL Server, could not try on mysql for now. Please let me know how it works.
CREATE TABLE mytable
(
number INT,
score INT
)
INSERT INTO mytable VALUES ( 1, 100)
INSERT INTO mytable VALUES ( 2, 100)
INSERT INTO mytable VALUES ( 2, 120)
INSERT INTO mytable VALUES ( 2, 110)
INSERT INTO mytable VALUES ( 3, 120)
INSERT INTO mytable VALUES ( 3, 150)
SELECT *
FROM mytable m
WHERE
(
SELECT COUNT(*)
FROM mytable m2
WHERE m2.number = m.number AND
m2.score >= m.score
) <= 2

Answer 3

How about this?
SELECT event_id, user_id
FROM (
SELECT event_id, user_id, row_number() OVER (PARTITION BY user_id) AS row_num
FROM EventUser WHERE user_id in (1,2)) WHERE row_num <= n;
And n can be whatever

Answer 4

Later but help uses a derived table and the cross join.
For the example in this post the query will be this:
SELECT
#row_number:=CASE
WHEN #user_no = user_id
THEN
#row_number + 1
ELSE
1
END AS num,
#user_no:=user_id userid, event_id
FROM
EventUser,
(SELECT #user_no:=0,#row_number:=0) as t
group by user_id,event_id
having num < 3;
More information in this link.