Question

HQL query to select first records in time interval.

Asked by: traverss

Hi

I have a situation where devices in the field send their readings and is persisted every hour. The devices measure consumption with a counter (cumulative). This counter reading is persisted every hour.
The persistant entity is as follows:

class Reading {
 int deviceId;
 int reading;
 Date timestamp;
}

I need to report on the total consumption of all the devices during a time interval (say, a day), and how many devices' reading i used.
To get an accurate report, for each device, the difference between the first reading (minimum reading) of the current time interval and the next time interval must be calculated.
I wrote a query which gives me the first reading (smallest reading ) for each device for a time interval. But now I need to get te second reading as well, which will be the smallest reading after the nextIntervalStart time. I don't know if a subquery is the answer?

Current query:

select r.deviceId, min(r.reading),
from Reading r
where r.timestamp >= :intervalStart
and r.timestamp < :nextIntervalStart
group by r.deviceId

It would be nice to have an HQL query which returns the device id, and 2 readings (the first reading of each interval per device).

Query result wanted:

deviceId, reading1, reading2
1, 2000, 2100
2, 1000, 1150
3, 1560, 1800

Or a query which returns device id and consumption delta.

deviceId, consumption
1, 100
2, 150
3, 240

Thanks, any help will be appreciated.

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Asked On
2008-09-09 at 00:06:30ID23714538
Tags

Hibernate, HQL

Topics

J2EE Frameworks

,

MySQL Server

Participating Experts
2
Points
500
Comments
7

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Answers

 

by: VenabiliPosted on 2008-09-09 at 03:15:13ID: 22425698

OK - so you need a query to find the difference between the total minimal value and the minimal value for only part of the column (the rows where the timestamp >nextIntervalStart? I'll see if I can come up with an idea here

All things considered - won't it actually be easier to run two different quieries and to do the extraction in the Java code?

 

by: traverssPosted on 2008-09-09 at 03:41:14ID: 22425851

Hi

Thanks for your help. Lets just get the problem 100% clear. Maybe my first description isn't entirely clear.

I need the minimum reading in the first interval (intervalStart) of each device and the minimum reading in the next interval (nextIntervalStart) for each device. Then getting the difference of the 2 might be easier...I hope.

As for doing it in Java, it's possible. I'll just have to construct something like a hashMap to keep the readings of each device together where the deviceId is the key in the map and a list of readings as the values. In the end, I'm going to run this query for a number of successive intervals (intervalStart := nextIntervalStart and nextIntervalStart := nextIntervalStart + someInterval)  So I'll have the delta consumption per device for each interval.

I thought it would be quicker if the db does the processing?

Kind regards

 

by: angelIIIPosted on 2008-09-09 at 07:42:54ID: 22427886

could you please clarify with data samples?
also, with "first" reading vs "minimum" reading there might be some different interpretations, can you clarify exactly, please?
I understand with "first" the one with the lowest datetimestamp value in the interval...
I understand with "minimum" the one with the lowest reading value in the interval...

please also the version of MySQL...

 

by: traverssPosted on 2008-09-10 at 01:22:10ID: 22436257

Ok, typical records that will be in the table:

deviceID, reading, timestamp (yyyy-MM-dd hh:mm)
1, 4000, 2008-09-09 10:00
2, 2500, 2008-09-09 10:09
1, 4500, 2008-09-09 11:00
2, 3100, 2008-09-09 11:08
1, 4900, 2008-09-09 12:00
2, 2500, 2008-09-09 12:09
.
.
.
1, 9700, 2008-09-10 04:00
2, 8500, 2008-09-10 04:09
1, 10300, 2008-09-10 05:01
2, 9500, 2008-09-10 05:07
etc.

Because the readings are only ever increasing, the first reading in an interval (one with the lowest datetimestamp) should also be the one with the lowest reading for a particular device in the interval. I used min(reading) in my samle query because i don't think min() works on a date field. Selecting the "first" reading in the interval by date would be semantically correct.

Sometimes the reading might not change for consecutive readings. (nothing is consumed in the period).

I'm using MySQL 5.0.51b and Hibernate 3.2.6 ga.

If a Hibernate query is not possible, I'll consider doing direct SQL, but that would hinder portability. There is a high likelihood that we'll switch DBs later.

 

by: angelIIIPosted on 2008-09-10 at 02:14:57ID: 22436513

ok, let's see if this pleases you.
which is, btw, some syntax that should work in all the (real) databases I work with regularly:
ms sql server, oracle, mysql

select r.deviceId
, min(r.reading) first_reading
, max(r.reading) next_reading
, max(r.reading) - min(r.reading) consumption
from Reading r
where r.timestamp = ( select min(i.timestamp) from Reading i
                       where i.deviceID = r.deviceID 
                         and i.timestamp >= :intervalStart
                         and i.timestamp < :nextIntervalStart
                     )
   or r.timestamp = ( select min(i.timestamp) from Reading i
                       where i.deviceID = r.deviceID 
                         and i.timestamp >= :nextIntervalStart
                     )
group by r.deviceId

                                              
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by: traverssPosted on 2008-09-10 at 02:40:16ID: 22436639

Great, thanks. I'll investigate this and report back.

 

by: traverssPosted on 2008-09-10 at 23:45:01ID: 31494589

Thanks for the help.

20120131-EE-VQP-002

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