Prova questo:
SELECT r.id,
r.`date`,
GROUP_CONCAT(IF(r.device_id = 1,r.`VALUE`,NULL)) AS device_id_1,
GROUP_CONCAT(IF(r.device_id = 2,r.`VALUE`,NULL)) AS device_id_2
FROM readings r
GROUP BY r.`DATE`
ORDER BY r.`DATE` ASC;
+----+---------------------+-------------+-------------+
| id | date | device_id_1 | device_id_2 |
+----+---------------------+-------------+-------------+
| 1 | 2015-10-01 10:00:00 | 2 | 0.5 |
| 3 | 2015-10-01 10:01:00 | 3.1 | 7.5 |
+----+---------------------+-------------+-------------+
2 rows in set (0.00 sec)
E qui una versione per device_id dinamici
SELECT group_concat(
"GROUP_CONCAT(IF(device_id = ",
device_id,",`value`,NULL)) AS device_id_",
device_id
SEPARATOR ',\n') INTO @sql_mid
FROM (SELECT DISTINCT device_id FROM readings ORDER BY device_id) AS r
ORDER BY device_id;
SELECT CONCAT("SELECT id, `date`, ", @sql_mid, " FROM readings GROUP BY `DATE` ORDER BY `DATE` ASC") INTO @sql;
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
Genera tutte le colonne
+----+---------------------+-------------+-------------+-------------+
| id | date | device_id_1 | device_id_2 | device_id_3 |
+----+---------------------+-------------+-------------+-------------+
| 1 | 2015-10-01 10:00:00 | 2 | 0.5 | NULL |
| 3 | 2015-10-01 10:01:00 | 3.1 | 7.5 | NULL |
| 5 | 2015-10-01 10:11:00 | NULL | NULL | 9.9 |
+----+---------------------+-------------+-------------+-------------+
3 rows in set (0.00 sec)
