DATE lookup table (1990/01/01:2041/12/31)

问题:

I use a DATE's master table for looking up dates and other values in order to control several events, intervals and calculations within my app. It has rows for every single day begining from 01/01/1990 to 12/31/2041.

One example of how I use this lookup table is:

1. A customer pawned an item on: JAN-31-2010
2. Customer returns on MAY-03-2010 to make an interest pymt to avoid forfeiting the item.
3. If he pays 1 months interest, the employee enters a "1" and the app looks-up the pawn
   date (JAN-31-2010) in date master table and puts FEB-28-2010 in the applicable interest pymt date. FEB-28 is returned because FEB-31's dont exist! If 2010 were a leap-year, it would've returned FEB-29.
4. If customer pays 2 months, MAR-31-2010 is returned. 3 months, APR-30... If customer pays more than 3 months or another period not covered by the date lookup table, employee manually enters the applicable date.

Here's what the date lookup table looks like:

---

{ Copyright 1990:2010, Frank Computer, Inc. }

{ DBDATE=YMD4- (correctly sorted for faster lookup) }

CREATE TABLE     datemast 
(
 dm_lookup       DATE,    {lookup col used for obtaining values below}
 dm_workday      CHAR(2), {NULL=Normal Working Date,}
                          {NW=National Holiday(Working Date),}
                          {NN=National Holiday(Non-Working Date),}
                          {NH=National Holiday(Half-Day Working Date),}
                          {CN=Company Proclamated(Non-Working Date),}
                          {CH=Company Proclamated(Half-Day Working Date)}

 {several other columns omitted}

 dm_description CHAR(30), {NULL, holiday description or any comments}
 dm_day_num     SMALLINT, {number of elapsed days since begining of year}
 dm_days_left   SMALLINT, (number of remaining days until end of year}

 dm_plus1_mth   DATE,     {plus 1 month from lookup date}
 dm_plus2_mth   DATE,     {plus 2 months from lookup date}
 dm_plus3_mth   DATE,     {plus 3 months from lookup date}
 dm_fy_begins   DATE,     {fiscal year begins on for lookup date}
 dm_fy_ends     DATE,     {fiscal year ends on for lookup date}
 dm_qtr_begins  DATE,     {quarter begins on for lookup date}
 dm_qtr_ends    DATE,     {quarter ends on for lookup date}
 dm_mth_begins  DATE,     {month begins on for lookup date}
 dm_mth_ends    DATE,     {month ends on for lookup date}
 dm_wk_begins   DATE,     {week begins on for lookup date}
 dm_wk_ends     DATE,     {week ends on for lookup date}

 {several other columns omitted}
)
IN "S:\PAWNSHOP.DBS\DATEMAST"; 

Is there a better way of doing this or is it a cool method?

回答1:

This is a reasonable way of doing things. If you look into data warehousing, you'll find that those systems often use a similar system for the time fact table. Since there are less than 20K rows in the fifty-year span you're using, there isn't a huge amount of data.

There's an assumption that the storage gives better performance than doing the computations; that most certainly isn't clear cut since the computations are not that hard (though neither are they trivial) and any disk access is very slow in computational terms. However, the convenience of having the information in one table may be sufficient to warrant having to keep track of an appropriate method for each of the computed values stored in the table.

回答2:

It depends on which database you are using. SQL Server has horrible support for temporal data and I almost always end up using a date fact table there. But databases like Oracle, Postgres and DB2 have really good support and it is typically more efficient to calculate dates on the fly for OLTP applications.

For instance, Oracle has a last_day() function to get the last day of a month and an add_months() function to, well, add months. Typically in Oracle I'll use a pipelined function that takes start and end dates and returns a nested table of dates.

回答3:

The cool way of generating a rowset of dates in Oracle is to use the hierarchical query functionality, connect by. I have posted an example of this usage in another thread.

It gives a lot of flexibility without the PL/SQL overhead of a pipelined function.

回答4:

OK, so I tested my app using 31 days/month to calculate interest rates & pawnshops are happy with it! Local Law prays as follows: From pawn or last int. pymt. date to 5 elapsed days, 5% interest on principal, 6 to 10 days = 10%, 11 to 15 days = 15%, and 16 days to 1 "month" = 20%.

So the interest table is now defined as follows:

NUMBER OF ELAPSED DAYS SINCE
PAWN DATE OR LAST INTEREST PYMT

 FROM     TO  ACUMULATED
  DAY    DAY    INTEREST
-----   ----  ----------
    0      5       5.00%
    6     10      10.00%
   11     15      15.00%
   16     31      20.00%
   32     36      25.00%
   37     41      30.00%
   42     46      35.00%
   47     62      40.00%

   [... until day 90 (forfeiture allowed)]
   from day 91 to 999, daily prorate based on 20%/month.

Did something bad happen in the UK on MAR-13 or SEP-1752?