Using LIMIT within GROUP BY to get N results per group?
SELECT
year, id, rate
FROM h
WHERE year BETWEEN 2000 AND 2009
AND id IN (SELECT rid FROM table2)
GROUP BY id, year
ORDER BY id, rate DESC
yields:
year id rate
2006 p01 8
2003 p01 7.4
2008 p01 6.8
2001 p01 5.9
2007 p01 5.3
2009 p01 4.4
2002 p01 3.9
2004 p01 3.5
2005 p01 2.1
2000 p01 0.8
2001 p02 12.5
2004 p02 12.4
2002 p02 12.2
2003 p02 10.3
2000 p02 8.7
2006 p02 4.6
2007 p02 3.3
What I'd like is only the top 5 results for each id:
2006 p01 8
2003 p01 7.4
2008 p01 6.8
2001 p01 5.9
2007 p01 5.3
2001 p02 12.5
2004 p02 12.4
2002 p02 12.2
2003 p02 10.3
2000 p02 8.7
Is there a way to do this using some kind of LIMIT like modifier that works within the GROUP BY?
2Answer
This can be done in MySQL, but it is not as simple as adding a LIMIT clause. Here is an article that explains the problem in detail:
How to select the first/least/max row per group in SQL
It's a good article - he introduces an elegant but naïve solution to the "Top N per group" problem, and then gradually improves on it.
- answered 8 years ago
- G John
You could use GROUP_CONCAT aggregated function to get all years into a single column, grouped by id and ordered by rate:
SELECT id, GROUP_CONCAT(year ORDER BY rate DESC) grouped_year
FROM yourtable
GROUP BY id
Result:
-----------------------------------------------------------
| ID | GROUPED_YEAR |
-----------------------------------------------------------
| p01 | 2006,2003,2008,2001,2007,2009,2002,2004,2005,2000 |
| p02 | 2001,2004,2002,2003,2000,2006,2007 |
-----------------------------------------------------------
And then you could use FIND_IN_SET, that returns the position of the first argument inside the second one, eg.
SELECT FIND_IN_SET('2006', '2006,2003,2008,2001,2007,2009,2002,2004,2005,2000');
1
SELECT FIND_IN_SET('2009', '2006,2003,2008,2001,2007,2009,2002,2004,2005,2000');
6
Using a combination of GROUP_CONCAT and FIND_IN_SET, and filtering by the position returned by find_in_set, you could then use this query that returns only the first 5 years for every id:
SELECT
yourtable.*
FROM
yourtable INNER JOIN (
SELECT
id,
GROUP_CONCAT(year ORDER BY rate DESC) grouped_year
FROM
yourtable
GROUP BY id) group_max
ON yourtable.id = group_max.id
AND FIND_IN_SET(year, grouped_year) BETWEEN 1 AND 5
ORDER BY
yourtable.id, yourtable.year DESC;
- answered 8 years ago
- G John
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