# Working with periodical data

If data is received periodically because it is provided on a change of valve or just because measuring times are sporadic, how is data be queried with that falls between measurements?

Or through example, how can I determine the average flow over the range from 2017-07-29 00:00:00 to 2017-08-03 00:00:00?

``````Time	            Flow
2017-07-25 15:38:04	40.43
2017-07-28 21:09:51	43.50
2017-08-04 02:52:31	47.14
2017-08-08 00:40:36	47.66
2017-08-11 06:50:59	47.27
2017-08-14 17:43:02	50.18
2017-08-17 16:00:07	48.76
2017-08-21 10:42:56	44.20``````

As shown, it is not possible to do so. True? I expect query durations must always be above a given span which the application that writes data ensure that records are inserted within that duration. True? Thank you

``````> SELECT value FROM "point_74" WHERE time >= '2017-07-30T02:35:00Z' AND time <= '2017-07-30T02:45:00Z'
name: point_74
time                 value
----                 -----
2017-07-30T02:35:49Z 696.73
2017-07-30T02:36:49Z 696.748
2017-07-30T02:37:50Z 696.768
2017-07-30T02:38:50Z 696.785
2017-07-30T02:39:50Z 696.803
2017-07-30T02:40:50Z 696.822
2017-07-30T02:41:51Z 696.84
2017-07-30T02:42:51Z 696.857
2017-07-30T02:43:51Z 696.877
2017-07-30T02:44:51Z 696.895
> SELECT MEAN("value") FROM "point_74" WHERE time >= '2017-07-30T02:42:51.1Z' AND time <= '2017-07-30T02:42:51.9Z'
> SELECT MEAN("value") FROM "point_74" WHERE time >= '2017-07-30T02:42:50.9Z' AND time <= '2017-07-30T02:42:51.1Z'
name: point_74
time                   mean
----                   ----
2017-07-30T02:42:50.9Z 696.857
>``````

Or based on example data from the following, how can I query the water level in santa_monica at 2015-09-18T21:40:00Z?
EDIT. Guess this can be performed with LAST().

``````> SELECT * FROM "h2o_feet"

name: h2o_feet
--------------
time                   level description      location       water_level
2015-08-18T00:00:00Z   below 3 feet           santa_monica   2.064
2015-08-18T00:00:00Z   between 6 and 9 feet   coyote_creek   8.12
[...]
2015-09-18T21:36:00Z   between 3 and 6 feet   santa_monica   5.066
2015-09-18T21:42:00Z   between 3 and 6 feet   santa_monica   4.938``````