Re: [libreoffice-users] Other SQL question as it affects Base

El 30/07/12 2:08, Dan escribió:
> Mark Stanton wrote:
> > Keys are intended for the internal workings of the database, they are
> > not meant to be
> > used like this.
> >
> > What you want is the WHERE clause, or occasionally the HAVING clause,
> > matching or
> > excluding rows based on their data.
> >
> > Keys are NOT data and should not be used as such.
> >
> > Regards Mark Stanton One small step for mankind...
>
>       I have a table, Reading, that contains three fields: ID, Date, and
> Meter. Date is the
> date the meter is read, and Meter is the meter reading.
>       I want to know how much electricity (KWH used) I have used each
> month. The query below will provide that information:
>
> SELECT "Ending"."Date", "Ending"."Meter" -"Beginning"."Meter" AS "KWH
> Used" FROM "Power". "Reading" AS "Beginning" LEFT JOIN "Power".
> "Reading" AS" Ending" ON "Ending"."ID" =1 + "Beginning"."ID"
>
>       Perhaps you are right about keys being intended for the internal
> workings of the database. While I don't have an example of one key being
> the multiple of the other key, it is possible. It just might not have
> any practical applications.
>
> --Dan

I have used a sentence like next with Firebird, but I don't know if your 
database lets to use SELECT like a field.

SELECT R_End.Date,
         (SELECT R_Beg.Date FROM Reading AS R_Beg WHERE R_Beg.Date
           = (SELECT DISTINCT MAX(R_Look.Date) FROM Reading AS R_Look
               WHERE (R_Look.Date < R_End.Date))) AS Beginning_Date
       R_End.Meter, (R_End.Meter -
         (SELECT R_Beg.Meter FROM Reading AS R_Beg WHERE R_Beg.Date
           = (SELECT DISTINCT MAX(R_Look.Date) FROM Reading AS R_Look
               WHERE (R_Look.Date < R_End.Date)))) AS KwH_used
 FROM Reading as R_End


(SELECT DISTINCT MAX(R_Look.Date) FROM Reading AS R_Look
               WHERE (R_Look.Date < R_End.Date))
Search the maximum date before the actual row date, which lets to get 
the previous values to the actual row date.

Miguel Ángel.

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