IN MYSQL how to pick up the largest count each row? - mysql

Question

In mysql. I want to get some the largest counted review_id and mention.
below table is raw data
>> table Named: topic_id_review_id
id topic_id review_id
+-------+-------------+-------------+
1 1 1
2 1 1
3 1 1
4 2 1
5 4 1
6 3 2
7 5 2
8 2 3
9 2 3
10 3 3
11 3 1
12 5 4
13 5 1
14 6 5
16 7 7
17 7 7
18 7 7
...
....
**I want to get the top of four(4) topics about each of review_id.
>> wanted!!
topic_id review_id
+-------------+-------------+
1 1
2 1
3 1
4 1
3 2
5 2
2 3
3 3
5 4
...
....
like above table, I want to top 4 topics in Every review_id.
I have already tried some code. but all of thing is not good results...
SELECT
tr1.topic_id, tr1.review_id
FROM topic_id_review_id tr1
WHERE review_id IN (1,2,3)AND(
SELECT COUNT(*)FROM topic_id_review_id tr2
WHERE tr2.review_id = tr1.review_id) < 4;
SELECT
COUNT(*) AS `count`
FROM topic_id_review_id tr2
WHERE tr2.review_id = 1
GROUP BY topic_id, review_id
order by `count` DESC LIMIT 4;
maybe your guy, this not difficult... but as beginner to me...
it is not simple. so please let me know to to solve it! thanks

Answer 1

One way that works in all versions of MySql is the use of a correlated subquery in the WHERE clause which counts the number of topic_ids for each review_id:
select
t.topic_id, t.review_id
from (
select min(id) id, topic_id, review_id
from topic_id_review_id
group by topic_id, review_id
) t
where (
select count(*) from topic_id_review_id
where review_id = t.review_id and topic_id <= t.topic_id and id < t.id
) <= 4
order by t.review_id, t.topic_id
See the demo.
For MySql 8.0+:
with cte as (
select
t.topic_id, t.review_id,
row_number() over (partition by t.review_id order by t.topic_id, t.id) rn
from (
select min(id) id, topic_id, review_id
from topic_id_review_id
group by topic_id, review_id
) t
)
select
c.topic_id, c.review_id
from cte c
where c.rn <= 4
order by c.review_id, c.topic_id
See the demo.
Results:
| topic_id | review_id |
| -------- | --------- |
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 1 |
| 3 | 2 |
| 5 | 2 |
| 2 | 3 |
| 3 | 3 |
| 5 | 4 |
| 6 | 5 |
| 7 | 7 |