jono2010 jono2010 - 16 days ago 5
MySQL Question

MYSQL - Group by limit

Is there a simple way to LIMIT the GROUP BY results to the top 2. The following query returns all the results. Using 'LIMIT 2' reduces the overall list to the top 2 entries only.

select distinct(rating_name),
id_markets,
sum(rating_good) 'good',
sum(rating_neutral)'neutral',
sum(rating_bad) 'bad'
from ratings
where rating_year=year(curdate()) and rating_week= week(curdate(),1)
group by rating_name,id_markets
order by rating_name, sum(rating_good)
desc


Results in the following :-


poland 78 48 24 12 <- keep
poland 1 15 5 0 <- keep
poland 23 12 6 3
poland 2 5 0 0
poland 3 0 5 0
poland 4 0 0 5
ireland 1 9 3 0 <- keep
ireland 2 3 0 0 <- keep
ireland 3 0 3 0
ireland 4 0 0 3
france 12 24 12 6 <- keep
france 1 3 1 0 <- keep
france 231 1 0 0
france 2 1 0 0
france 4 0 0 1
france 3 0 1 0


Thanks
Jon




As requested I have attached a copy of the table structure and some test data. My goal is to create a single view that has the top 2 results from each unique rating_name

CREATE TABLE `zzratings` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`id_markets` int(11) DEFAULT NULL,
`id_account` int(11) DEFAULT NULL,
`id_users` int(11) DEFAULT NULL,
`dateTime` timestamp NULL DEFAULT CURRENT_TIMESTAMP,
`rating_good` int(11) DEFAULT NULL,
`rating_neutral` int(11) DEFAULT NULL,
`rating_bad` int(11) DEFAULT NULL,
`rating_name` varchar(32) DEFAULT NULL,
`rating_year` smallint(4) DEFAULT NULL,
`rating_week` tinyint(4) DEFAULT NULL,
`cash_balance` decimal(9,6) DEFAULT NULL,
`cash_spend` decimal(9,6) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `rating_year` (`rating_year`),
KEY `rating_week` (`rating_week`),
KEY `rating_name` (`rating_name`)
) ENGINE=MyISAM AUTO_INCREMENT=2166690 DEFAULT CHARSET=latin1;

INSERT INTO `zzratings` (`id`,`id_markets`,`id_account`,`id_users`,`dateTime`,`rating_good`,`rating_neutral`,`rating_bad`,`rating_name`,`rating_year`,`rating_week`,`cash_balance`,`cash_spend`)
VALUES
(63741, 1, NULL, 100, NULL, 1, NULL, NULL, 'poland', 2010, 15, NULL, NULL),
(63742, 1, NULL, 101, NULL, 1, NULL, NULL, 'poland', 2010, 15, NULL, NULL),
(1, 2, NULL, 102, NULL, 1, NULL, NULL, 'poland', 2010, 15, NULL, NULL),
(63743, 3, NULL, 103, NULL, NULL, 1, NULL, 'poland', 2010, 15, NULL, NULL),
(63744, 4, NULL, 104, NULL, NULL, NULL, 1, 'poland', 2010, 15, NULL, NULL),
(63745, 1, NULL, 105, NULL, 1, NULL, NULL, 'poland', 2010, 15, NULL, NULL),
(63746, 1, NULL, 106, NULL, NULL, 1, NULL, 'poland', 2010, 15, NULL, NULL),
(63747, 5, NULL, 100, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63748, 5, NULL, 101, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63749, 2, NULL, 102, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63750, 3, NULL, 103, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
(63751, 4, NULL, 104, NULL, NULL, NULL, 1, 'ireland', 2010, 15, NULL, NULL),
(63752, 1, NULL, 105, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63753, 1, NULL, 106, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
(63754, 1, NULL, 100, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63755, 1, NULL, 101, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63756, 2, NULL, 102, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63757, 34, NULL, 103, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
(63758, 34, NULL, 104, NULL, NULL, NULL, 1, 'ireland', 2010, 15, NULL, NULL),
(63759, 34, NULL, 105, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63760, 34, NULL, 106, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
(63761, 21, NULL, 100, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63762, 21, NULL, 101, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63763, 21, NULL, 102, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63764, 21, NULL, 103, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
(63765, 4, NULL, 104, NULL, NULL, NULL, 1, 'ireland', 2010, 15, NULL, NULL),
(63766, 1, NULL, 105, NULL, 1, NULL, NULL, 'ireland', 2010, 15, NULL, NULL),
(63767, 1, NULL, 106, NULL, NULL, 1, NULL, 'ireland', 2010, 15, NULL, NULL),
(63768, 1, NULL, 100, NULL, 1, NULL, NULL, 'france', 2010, 15, NULL, NULL),
(63769, 1, NULL, 101, NULL, 1, NULL, NULL, 'france', 2010, 15, NULL, NULL),
(63770, 2, NULL, 102, NULL, 1, NULL, NULL, 'france', 2010, 15, NULL, NULL),
(63771, 3, NULL, 103, NULL, NULL, 1, NULL, 'france', 2010, 15, NULL, NULL),
(63772, 4, NULL, 104, NULL, NULL, NULL, 1, 'france', 2010, 15, NULL, NULL);

Answer

I don't think that there is a simple way in MySQL. One way to do this is by generating a row number for each row partitioned in groups by rating_name, and then only select the rows with row_number 2 or less. In most databases you could do this using something like:

SELECT * FROM (
    SELECT
        rating_name,
        etc...,
        ROW_NUMBER() OVER (PARTITION BY rating_name ORDER BY good) AS rn
    FROM your_table
) T1
WHERE rn <= 2

Unfortunately, MySQL doesn't support the ROW_NUMBER syntax. You can however simulate ROW_NUMBER using variables:

SELECT
    rating_name, id_markets, good, neutral, bad
FROM (
    SELECT
        *,
        @rn := CASE WHEN @prev_rating_name = rating_name THEN @rn + 1 ELSE 1 END AS rn,
        @prev_rating_name := rating_name
    FROM (
        SELECT
            rating_name,
            id_markets,
            SUM(COALESCE(rating_good, 0)) AS good,
            SUM(COALESCE(rating_neutral, 0)) AS neutral,
            SUM(COALESCE(rating_bad, 0)) AS bad
        FROM zzratings
        WHERE rating_year = YEAR(CURDATE()) AND rating_week = WEEK(CURDATE(), 1)
        GROUP BY rating_name, id_markets
    ) AS T1, (SELECT @prev_rating_name := '', @rn := 0) AS vars
    ORDER BY rating_name, good DESC
) AS T2
WHERE rn <= 2
ORDER BY rating_name, good DESC

Result when run on your test data:

france    1  2  0  0
france    2  1  0  0
ireland   1  4  2  0
ireland  21  3  1  0
poland    1  3  1  0
poland    2  1  0  0
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