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Posted by --CELKO-- on 04/21/07 15:32
The usual design error is to have only one time in a row to capture
when an event started, then do horrible self-joins to get the duration
of the status change. Let me use a history table for price changes.
The fact to store is that a price had a duration:
CREATE TABLE PriceHistory
(upc CHAR(13) NOT NULL
REFERENCES Inventory(upc),
start_date DATE NOT NULL,
end_date DATE, -- null means current
CHECK(start_date < end_date),
PRIMARY KEY (upc, start_date),
item_price DECIMAL (12,4) NOT NULL
CHECK (item_price > 0.0000),
etc.);
You actually needs more checks to assure that the start date is at
00:00 and the end dates is at 23:59:59.999 Hrs. You then use a
BETWEEN predicate to get the appropriate price.
SELECT ..
FROM PriceHistory AS H, Orders AS O
WHERE O.sales_date BETWEEN H.start_date
AND COALESCE (end_date, CURRENT_TIMESTAMP);
It is also a good idea to have a VIEW with the current data:
CREATE VIEW CurrentPrices (..)
AS
SELECT ..
FROM PriceHistory
WHERE end_date IS NULL;
Look up the Rick Snodgrass book on Temporal Queries in SQL at the
University of Arizona website; it is a free download.
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