Can't get appropriate values from query? - mysql

Question

I want to list companyIds and with the mostly occur commentable type (0,1,2).
This is subquery
select a.companyId, a.commentable, count(1) _count
from article a
group by a.companyId, a.commentable
| companyId | commentable | _count |
|-----------|-------------|--------|
| 1 | 0 | 1 |
| 1 | 1 | 1 |
| 2 | 0 | 7759 |
| 2 | 1 | 7586 |
| 2 | 2 | 7856 |
| 3 | 0 | 7828 |
| 3 | 1 | 7866 |
| 3 | 2 | 7706 |
| 4 | 0 | 7851 |
| 4 | 1 | 7901 |
| 4 | 2 | 7738 |
| 5 | 0 | 7775 |
| 5 | 1 | 7884 |
| 5 | 2 | 7602 |
| 25 | 0 | 7888 |
| 25 | 1 | 7939 |
| 25 | 2 | 7784 |
For example above
Most commentable type occur for companyId=4 is 7901 and commentable type for that is 1. In below query , i see 4-0-7901, but i expected 4-1-7901
SELECT x.companyId, x.commentable, MAX(x._count) _count
FROM
( SELECT a.companyId, a.commentable, COUNT(1) _count
FROM article a
GROUP BY a.companyId, a.commentable
) AS X
GROUP BY x.companyId;
companyId commentable _count
1 0 1
2 0 7856
3 0 7866
4 0 7901
5 0 7884
25 0 7939
Expected result
companyId commentable _count
1 0 1
2 2 7856
3 1 7866
4 1 7901
5 1 7884
25 1 7939
I dont understand 'why is all commentable column is '0' .

Answer 1

You need a big ugly join here. In the query below, you may view the GROUP BY query on the company and comment type the base unit of work. This query appears as itself, aliased as t1. In alias t2, we subquery and aggregate only by commentable, to find the max count for each such comment type. This, we join back to t1 to restrict only the company having the max count.
SELECT
t1.companyId,
t1.commentable,
t1.cnt
FROM
(
SELECT companyId, commentable, COUNT(*) cnt
FROM article
GROUP BY companyId, commentable
) t1
INNER JOIN
(
SELECT companyId, MAX(cnt) max_cnt
FROM
(
SELECT companyId, commentable, COUNT(*) cnt
FROM article
GROUP BY companyId, commentable
) t
GROUP BY companyId
) t2
ON t1.companyId = t2.companyId AND t1.cnt = t2.max_cnt;
By the way, things get somewhat nicer in MySQL 8+, where we can take advantage of analytic functions:
WITH cte AS (
SELECT companyId, commentable, COUNT(*) cnt,
ROW_NUMBER() OVER (PARTITION BY commentable ORDER BY COUNT(*) DESC) rn
FROM article
GROUP BY companyId, commentable
)
SELECT companyId, commentable, cnt
FROM cte
WHERE rn = 1;

Answer 2

You can do this using a having clause:
SELECT a.companyId, a.commentable, COUNT(*) as _count
FROM article a
GROUP BY a.companyId, a.commentable
HAVING COUNT(*) = (SELECT COUNT(*)
FROM article a2
WHERE a2.companyId = a.companyId
GROUP BY a2.commentable
ORDER BY COUNT(*) DESC
LIMIT 1
);
In the event of ties, you will get multiple rows. If you want only one row per company, you can instead use commentable for the comparison in the HAVING:
SELECT a.companyId, a.commentable, COUNT(*) as _count
FROM article a
GROUP BY a.companyId, a.commentable
HAVING a.commentable = (SELECT a2.commentable
FROM article a2
WHERE a2.companyId = a.companyId
GROUP BY a2.commentable
ORDER BY COUNT(*) DESC
LIMIT 1
);
As others have mentioned, your problem is the mis-use of GROUP BY. The unaggregated columns in the SELECT need to match the GROUP BY keys -- and vice versa.

Answer 3

Cause commentable is not one of group by columns. In this case, with ONLY_FULL_GROUP_BY disabled, MySQL is free to choose any one value for this column.
From MySQL doc
If ONLY_FULL_GROUP_BY is disabled, a MySQL extension to the standard SQL use of GROUP BY permits the select list, HAVING condition, or ORDER BY list to refer to nonaggregated columns even if the columns are not functionally dependent on GROUP BY columns. This causes MySQL to accept the preceding query. In this case, the server is free to choose any value from each group, so unless they are the same, the values chosen are nondeterministic, which is probably not what you want.