问题描述
PIVOTing既不容易（也不是很好）。我更喜欢使用CASE：
SELECT d.data_timestamp
, SUM( CASE WHEN data_id = 1 THEN data_value ELSE 0 END ) AS 'input_1'
, SUM( CASE WHEN data_id = 2 THEN data_value ELSE 0 END ) AS 'input_2'
...
, SUM( CASE WHEN data_id = 20 THEN data_value ELSE 0 END ) AS 'input_20'
FROM data
GROUP BY data_timestamp
ORDER BY data_timestamp ASC
但也IF可以在MysqL中使用：
SELECT d.data_timestamp
, SUM( IF(data_id = 1, data_value, 0) ) AS 'input_1'
, SUM( IF(data_id = 2, data_value, 0) ) AS 'input_2'
...
, SUM( IF(data_id = 20, data_value, 0) ) AS 'input_20'
FROM data
GROUP BY data_timestamp
ORDER BY data_timestamp ASC
另外，您可以使用20级JOIN：
SELECT d.data_timestamp
, d01.data_value AS 'input_1'
, d02.data_value AS 'input_2'
...
, d20.data_value AS 'input_20'
FROM
( SELECT disTINCT d.data_timestamp
FROM data
) AS d
LEFT JOIN data AS d01
ON d01.data_timestamp = d.data_timestamp
AND d01.data_id = 1
LEFT JOIN data AS d02
ON d02.data_timestamp = d.data_timestamp
AND d02.data_id = 2
... --- 20 JOINs
LEFT JOIN data AS d20
ON d20.data_timestamp = d.data_timestamp
AND d20.data_id = 20
ORDER BY d.data_timestamp ASC
解决方法
我有一张要导出到CSV数据的表。理想情况下，我想在行和列之间进行切换，以使数据分组更好。
为了进一步解释，当前，数据库看起来像这样。
data_id data_timestamp data_value
--------------------------------------------
1 2011-07-07 00:01:00 0.400
1 2011-07-07 00:02:00 0.500
1 2011-07-07 00:03:00 0.600
1 2011-07-07 00:04:00 0.700
2 2011-07-07 00:01:00 0.100
2 2011-07-07 00:02:00 0.200
2 2011-07-07 00:03:00 0.250
2 2011-07-07 00:04:00 2.300
我想按data_timestamp值对data_value进行分组，以便对时间戳进行分组，并且每个data_id的每个data_value都显示在列中，而不是行中。
data_timestamp input_1 input_2
--------------------------------------------
2011-07-07 00:01:00 0.400 0.100
2011-07-07 00:02:00 0.500 0.200
2011-07-07 00:03:00 0.600 0.250
2011-07-07 00:04:00 0.700 2.300
以下是我正在使用的查询…
SELECT d.data_timestamp,d.input_1,d.input_2
FROM (
SELECT data_timestamp,IF(data_id=1,data_value,NULL) AS 'input_1',IF(data_id=2,NULL) AS 'input_2' FROM data
) AS d ORDER BY data_timestamp ASC
但这不是我想要的，因为只要一个data_id没有值，现在就有NULL值。GROUP BY似乎也将data_value分组，这不是我想要的。
有什么建议么？
编辑：
我已经尝试在外部查询中使用WHERE d.input_1 IS NOT NULL，但是无法完全获得结果。
在哪里之前…
data_timestamp input_1 input_2
--------------------------------------------
2011-07-07 00:01:00 0.400 NULL
2011-07-07 00:01:00 NULL 0.100
2011-07-07 00:02:00 0.500 NULL
2011-07-07 00:02:00 NULL 0.200
2011-07-07 00:03:00 0.600 NULL
2011-07-07 00:03:00 NULL 0.250
2011-07-07 00:04:00 0.700 NULL
2011-07-07 00:04:00 NULL 2.300
添加WHERE d.input_1 IS NOT NULL将删除input_2的值。
data_timestamp input_1 input_2
--------------------------------------------
2011-07-07 00:01:00 0.400 NULL
2011-07-07 00:02:00 0.500 NULL
2011-07-07 00:03:00 0.600 NULL
2011-07-07 00:04:00 0.700 NULL
另外，实际上，我大约有20个ID进行分组，因此也不是将所有这些都视为OR的最佳主意。