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Mysql group_concat的反向应用实现（Mysql列转行）

2016-05-11 14:58 459 查看

本文参考：http://blog.chinaunix.net/uid-411974-id-3990697.html，示例具业务场景略作改动。

-- 气温采集表，每天每个整点的气温度数
CREATE TABLE temp (
id INT,
time1 INT,
time2 INT,
time3 INT,
time4 INT,
receive_date DATE
) ;

-- 初始化数据
INSERT INTO `temp` (`id`, `time1`, `time2`, `time3`, `time4`, `receive_date`) VALUES('1','10','15','25','16','2016-05-11');
INSERT INTO `temp` (`id`, `time1`, `time2`, `time3`, `time4`, `receive_date`) VALUES('2','9','14','26','15','2016-05-10');
INSERT INTO `temp` (`id`, `time1`, `time2`, `time3`, `time4`, `receive_date`) VALUES('3','8','13','27','14','2016-05-09');
INSERT INTO `temp` (`id`, `time1`, `time2`, `time3`, `time4`, `receive_date`) VALUES('4','7','12','28','13','2016-05-08');
INSERT INTO `temp` (`id`, `time1`, `time2`, `time3`, `time4`, `receive_date`) VALUES('5','6','11','29','12','2016-05-07');

问题：查询5月份温度高于15度的次数，sql如何写？

如果列表较多，如100列，sql如何写？

解决：使用cross join，具体用法可参考文章顶部的博客原文。

脚本如下：

SELECT
receive_date,
SUBSTRING_INDEX(
SUBSTRING_INDEX(sub_id, ',', seq),
',',
- 1
) sub_id,
seq
FROM
(SELECT
0 seq
UNION
SELECT
1 seq
UNION
SELECT
2 seq
UNION
SELECT
3 seq
UNION
SELECT
4 seq) sequence
CROSS JOIN
(SELECT
CONCAT(
p.time1,
',',
p.time2,
',',
p.time3,
',',
p.time4
) sub_id,
p.receive_date
FROM
temp p) temp2
WHERE seq BETWEEN 1
AND 4
ORDER BY receive_date,
seq ASC ;

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