Re: Help with calculations in a query

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Tom,

I definitely would like to do this the right way, and any help you can give
me is most appreciated. I haven't had any formal training in database
programming, most of what I've learned has been from trial and error, and it
does take up a lot of time, but at the same time it is the best way for me to
learn something. I will have to get back to you tomorrow with the table
names and the column names, as I don't have the database at home.

Thanks again,
Richard

"Tom Ellison" wrote:

> Dear Richard:
>
> Well, Richard, I'll tell you what I do know.
>
> I do know that I've tried various approaches to solving many database
> problems. From some successes and failures, I've come to the conclusion
> that the rules about how databases should be constructed are extremely
> valuable. And these rules say not to store any derived values in tables.
> The biggest reason for that is as follows: If you have a derived value, and
> any of the stored values that are components of that value are changed, then
> the derived value must immediately change as well.
>
> I have actually written the code that will make a derived value follow any
> database changes. It is about 5-10 times as much work as doing it
> correctly, and it tends to be unreliable. It is too easy to forget one of
> the things that might change and make the derived value incorrect.
>
> For example, when you do this, how will you handle the case that a user
> deletes the row containing the previous day's reading? How will you handle
> it if the user then re-enters that row, but with a different value?
>
> I'm telling you, it's a complex mess handling all these eventualities, if
> you write the system incorrectly. However, if you simply derive all the
> derived values at the moment you need them, then you won't have this
> problem.
>
> Everything in my 23 year's experience writing database software screams that
> this is a really bad approach.
>
> The kind of query you need is something I write several times a week, and
> have done so hundreds of times. In my own project, this would not take me
> 15 minutes. I say that not to brag (there are many who post answers here
> who have similar experience and capability) but to encourage you to learn
> the best skills and to apply them properly. Very soon you'll see that this
> becomes easy to handle, and that this is definitely the best way to handle
> the situation.
>
> As I see it, you're at a turning point in how you will develop as a database
> programmer. If you listen to good advise, and put it into practice, you'll
> be vastly better off.
>
> Perhaps others who are experienced and read this will drop in and lend a
> vote to what I'm telling you, so you'll see what I say is quite true, and
> important. Because what I'm advising you is not my personal preference, but
> a very common experience of virtually all who have advanced professionally
> in this field.
>
> If you'll expose for me the name of your table and the names of your
> columns, and perhaps a bit of sample data, I'll write the query to do this.
> I've done so dozens of times per year for 5 or 6 years now in this
> newsgroup, and have used this as a tool to teach how this is done to dozens
> of others, and with quite good success (I'm guessing more than 95%). You'll
> probably be glad you did!
>
> Tom Ellison
>
>
> "RWhittet" <RWhittet@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
> news:7944BC86-4067-43DD-9DD7-4BE6BA16465D@xxxxxxxxxxxxxxxx
> > Dear Tom,
> >
> > After talking about this I am wondering if there isn't a better approach
> > to
> > this. Lets say that I created an extra field in my table for each meter
> > reading, and used my calculated controls on the form to set the values of
> > these fields. I have played around with the set value macro before but I
> > haven't gotten it to work. If this is possible I think would rather go
> > this
> > route. The meters we use are pretty sophisticated, they are connected to
> > the
> > SCADA system, and we also have operators who record the readings daily.
> > Give
> > this some thought and let me know what you think and if you have any
> > experience with the set value macro I would appreciate that too.
> >
> > Thanks,
> > Richard
> >
> > "Tom Ellison" wrote:
> >
> >> Dear Richard:
> >>
> >> Something about this makes me vary leary. So, the meter is never out of
> >> order, and the reading never fails to take place?
> >>
> >> Still, the query can be written in different ways, and each will react
> >> differently if a reading is missing for any reason. But I'll go ahead
> >> anyway.
> >>
> >> I will use an INNER JOIN between two copies of the table. JOIN it ON the
> >> column that identifies each meter. Filter it with so one table has the
> >> reading date equal to Forms![Meter Usage]!Date and the other to the same
> >> date minus 1. That's at least a starting point.
> >>
> >> I don't know the names of your table and columns. Without that I cannot
> >> be
> >> more specific.
> >>
> >> Any meter for which the two readings are not both present will be omitted
> >> from these results. It doesn't matter what kind of failure may have
> >> produced the missing data.
> >>
> >> Frankly, I would have thought to prorate between the reading whenever
> >> there
> >> is a missing reading. That is, if the meter read 13 on the first of the
> >> month, and 22 on the fourth, then the consumption on the second, or the
> >> third, or the fourth would each be 3 units.
> >>
> >> Before pushing such an idea, I'd use queries to investigate where there
> >> is
> >> any history of a meter reading being missing. A simple aggregate of
> >> COUNT(*) for each meter, and a COUNT(*) of every DISTINCT Date value in
> >> the
> >> table would tell me if there are missing readings, and how common they
> >> are.
> >>
> >> Those who instruct us to perform tasks like this often mistake just how
> >> reliable their data is. I wouldn't ever argue such a point except where
> >> it
> >> is historically demonstrable. Once you can prove the necessity for a
> >> cautious approach, then management can be made ready to listen.
> >>
> >> I hope you won't think I'm being intrusive. This is really the voice of
> >> reason and experience.
> >>
> >> Tom Ellison
> >>
> >>
> >> "RWhittet" <RWhittet@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
> >> news:07391C11-852B-40FF-A9DA-047519B6D4A5@xxxxxxxxxxxxxxxx
> >> > Thanks for the quick responses. There will never be a day when the
> >> > meters
> >> > are not read. The only case of an unread meter would be if that
> >> > pipeline
> >> > was
> >> > taken offline for repairs. And, this has happened. The ultimate goal
> >> > is
> >> > to
> >> > create a montly sum of the total amount of water through each meter to
> >> > be
> >> > used for billing, budgeting, predictions, etc. But, my boss created a
> >> > spreadsheet years ago that has been used ever since, so I am trying to
> >> > duplicate it, in a sense. It is important that these calculations be
> >> > accurate. The user pulling the report will select the beginning and
> >> > ending
> >> > date. The report itself is designed as a monthly report.
> >> >
> >> > This is the code I have used in my form to show the previous meter
> >> > reading.
> >> >
> >> > =DLookUp("[South Recycle]","[Meter Usage]","[Date] = Forms![Meter
> >> > Usage]![Date]-1")
> >> >
> >> > Richard Whittet
> >> >
> >> > "Tom Ellison" wrote:
> >> >
> >> >> Dear Richard:
> >> >>
> >> >> Before helping prepare a way to query this, I want to check certain
> >> >> facts.
> >> >>
> >> >> You say, "These meters are read everyday" Does that mean it is never
> >> >> the
> >> >> case that a day goes by in which no meters are read (a holiday for
> >> >> example)?
> >> >> Does it mean that it is never the case that a meter is skipped?
> >> >>
> >> >> If a day is skipped, like a holiday, then are you going to give
> >> >> results
> >> >> for
> >> >> a 2 day period instead? If a meter is skipped, will you skip
> >> >> reporting
> >> >> that
> >> >> meter for 2 days (the day in which it has the current reading, then
> >> >> the
> >> >> day
> >> >> in which it has the previous reading).
> >> >>
> >> >> Will the user pulling this report select the Day 1 date? Will you
> >> >> then
> >> >> report only those meter that were read on the chosen date AND on the
> >> >> previous day?
> >> >>
> >> >> Realistically, real world data has holes in it. I'm trying to get our
> >> >> thinking together on whether such holes will EVER happen, and what you
> >> >> want
> >> >> to do in that case.
> >> >>
> >> >> In order to do what you want, you need to use two copies of the table
> >> >> in
> >> >> the
> >> >> same query. One feature of doing this is to alias them, so you can
> >> >> reference them separately.
> >> >>
> >> >> I need some detailed, precise information on the questions I asked
> >> >> before
> >> >> I
> >> >> can begin to craft any solution. OK?
> >> >>
> >> >> I'm going to suggest one possible solution to the problem. Let's say
> >> >> you
> >> >> have a control on a form with the more recent date in it. We could
> >> >> then
> >> >> build a query that filters to that date and to the immediately
> >> >> preceeding
> >> >> calendar date. Any meter that does not have BOTH readings would be
> >> >> excluded
> >> >> from the results. That's a simple, but perhaps not most desirable way
> >> >> to
> >> >> handle it.
> >> >>
> >> >> Tom Ellison
> >> >>
> >> >>
> >> >> "RWhittet" <RWhittet@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
> >> >> news:06106176-49F2-4CC5-9FBE-82844CCCA68E@xxxxxxxxxxxxxxxx
> >> >> >I have run into a problem in my query. I am trying to run
> >> >> >calculations
> >> >> >on
> >> >> > water meters. I have been able to calculate the information I need
> >> >> > on
> >> >> > my
> >> >> > forms, by using DLookup, but it's a bit more tricky to do in the
> >> >> > query.
> >> >> > These meters are read everyday, and the calculations needed are also
> >> >> > daily
> >> >> > as
> >> >> > they convert the value into million gallons per day. The
> >> >> > calculation
> >> >> > is
> >> >> > as
> >> >> > follows;
> >> >> > (Day 2 meter reading - Day 1 meter reading)/1000
> >> >> >
> >> >> > All of the readings are stored in the same table, so the biggest
> >> >> > problem
> >> >> > is
> >> >> > referencing the value from the previous record. I have also been
> >> >> > able
> >> >> > to
> >> >> > create the calculations in my reports, but I'm unable to use a
> >> >> > monthly
> >> >> > average or sum on a calculated field in a report. Any solutions or
> >> >> > suggestions would be greatly appreciated!
> >> >> >
> >> >> > Richard Whittet
> >> >>
> >> >>
> >> >>
> >>
> >>
> >>
>
>
>
.

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