influxDB---Data Exploration

Posted 2020-10-01 michellexiaoqi

1、The Group by clause and fill()

一般情况下，一个group by 间隔时间用null输出代表没有值的数据，可用fill()来改变没有值 的输出方式。fill()选项包括

• 任何数字
• null:设置null为间隔内没有数据的输出
• previous:复制前一个间隔的值作为没有数据的输出
• none:跳过没有值

以下的例子可以看出fill()的用法

GROUP BY without fill()

例子1 

SELECT MEAN(water_level) FROM h2o_feet WHERE time >= ‘2015-08-18‘ AND time < ‘2015-09-24‘ GROUP BY time(10d)
结果

name: h2o_feet
--------------
time			                 mean
2015-08-13T00:00:00Z	   4.306212083333323
2015-08-23T00:00:00Z	   4.318944629367029
2015-09-02T00:00:00Z	   4.363877681204781
2015-09-12T00:00:00Z   	　4.69811470811633
?2015-09-22T00:00:00Z

GROUP BY with fill()
例子2
Use fill() with -100:

SELECT MEAN(water_level) FROM h2o_feet WHERE time >= ‘2015-08-18‘ AND time < ‘2015-09-24‘ GROUP BY time(10d) fill(-100)

结果

name: h2o_feet
--------------
time			                 mean
2015-08-13T00:00:00Z	   4.306212083333323
2015-08-23T00:00:00Z	   4.318944629367029
2015-09-02T00:00:00Z	   4.363877681204781
2015-09-12T00:00:00Z	   4.698114708116322
?2015-09-22T00:00:00Z	   -100

例子3
Use fill() with none:

SELECT MEAN(water_level) FROM h2o_feet WHERE time >= ‘2015-08-18‘ AND time < ‘2015-09-24‘ GROUP BY time(10d) fill(none)
结果

name: h2o_feet
--------------
time			               mean
2015-08-13T00:00:00Z	 4.306212083333323
2015-08-23T00:00:00Z	 4.318944629367029
2015-09-02T00:00:00Z	 4.363877681204781
2015-09-12T00:00:00Z	 4.69811470811633
?

注意：如果你用GROUP(ing) BY一些东西（如，有tags和一上时间间隔），fill()必须放在group by 的最后。