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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