mysql GROUP_CONCAT+ GROUP BY + substring_index擷取分組的前幾名

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mysql方法來源於：http://www.cnblogs.com/jjcc/p/5896588.html

###在網上看到一篇，非常贊的方法

比如說要擷取班級的前3名，mysql就可以用GROUP_CONCAT  + GROUP BY + substring_index實現。

考試表

DROP TABLE IF EXISTS `test`;
CREATE TABLE `test` (
`id` int(11) DEFAULT NULL,
`name` varchar(20) DEFAULT NULL,
`score` int(11) DEFAULT NULL,
`class` char(12) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8; 

插入資料

INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘1‘, ‘Bobdd‘, ‘25‘, ‘1‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘2‘, ‘xx‘, ‘20‘, ‘2‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘3‘, ‘Jack‘, ‘30‘, ‘2‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘4‘, ‘Bill‘, ‘32‘, ‘4‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘5‘, ‘Nick‘, ‘22‘, ‘3‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘6‘, ‘Kathy‘, ‘18‘, ‘3‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘7‘, ‘Steve‘, ‘36‘, ‘3‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘8‘, ‘Anne‘, ‘25‘, ‘2‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘9‘, ‘Kathy‘, ‘18‘, ‘2‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘11‘, ‘Bob1‘, ‘25‘, ‘3‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘12‘, ‘Jane1‘, ‘20‘, ‘1‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘13‘, ‘Jack1‘, ‘30‘, ‘1‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘14‘, ‘Bill1‘, ‘32‘, ‘1‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘15‘, ‘Nick1‘, ‘22‘, ‘4‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘16‘, ‘Kathy1‘, ‘18‘, ‘4‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘17‘, ‘Steve1‘, ‘36‘, ‘4‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘18‘, ‘Anne1‘, ‘25‘, ‘1‘);
INSERT INTO `test` (`id`, `name`, `score`, `class`) VALUES (‘19‘, ‘Kathy1‘, ‘18‘, ‘2‘);

 

運用group_concat + GROUP BY 分組 擷取前3名

select GROUP_CONCAT(t1.id) as ids from (
SELECT t.class, substring_index(GROUP_CONCAT(t.id ORDER BY t.score desc),‘,‘,3) as id from
test t GROUP BY t.class 
)t1

得到

注意 是t.id ORDER BY t.score desc 分數從高到低。

上面的語句只是擷取到總的id。但是轉換為列不太好弄。可以拆分用union all 來搞。

擷取第一名

SELECT t.class, substring_index(GROUP_CONCAT(t.id ORDER BY t.score desc),‘,‘,1) as id from
test t GROUP BY t.class 

union all

-- 第二名
SELECT t.class, substring_index(substring_index(GROUP_CONCAT(t.id ORDER BY t.score desc),‘,‘,2),‘,‘,-1) as id from
test t GROUP BY t.class

union all

-- 第三名
SELECT t.class, substring_index(substring_index(GROUP_CONCAT(t.id ORDER BY t.score desc),‘,‘,3),‘,‘,-1) as id from
test t GROUP BY t.class

好了到現在 已經擷取到了一個list

用 in 來完成最後的步驟 

SELECT class,score,name FROM test where id in(
SELECT id from 
(SELECT t.class, substring_index(GROUP_CONCAT(t.id ORDER BY t.score desc),‘,‘,1) as id from
test t GROUP BY t.class 
union all  
SELECT t.class, substring_index(substring_index(GROUP_CONCAT(t.id ORDER BY t.score desc),‘,‘,2),‘,‘,-1) as id from
test t GROUP BY t.class 
union all 
SELECT t.class, substring_index(substring_index(GROUP_CONCAT(t.id ORDER BY t.score desc),‘,‘,3),‘,‘,-1) as id from
test t GROUP BY t.class) t2
) ORDER BY class asc,score desc

 最終結果

如果單純使用mysql來寫，太複雜，建議使用R :1,用order對score降序排列 2，df[,head(.SD,3), by=class] #對班級分組，返回每組的前三行

mysql GROUP_CONCAT+ GROUP BY + substring_index擷取分組的前幾名