Re: help yourself by helping others

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On Friday 04 April 2003 03:10, Ali Adams wrote:
> Dear All,
>
> I am new to Relational Databases and SQL and my background in ODBs is
> clouding my way to solving what seems to be a simple problem. I am sure
> many of you have met it many times.
>
> OK, I have a table as follows:
>
>
> ID Machine Date Withdrawals
> 1 1 01/01/2003 1101
> 2 2 01/01/2003 2101
> 3 3 01/01/2003 3101
>
> 4 1 02/01/2003 1102
> 5 2 02/01/2003 2102
>
> 6 1 03/01/2003 1103
> 7 3 03/01/2003 3103
>
> 8 2 04/01/2003 2104
> 9 4 04/01/2003 4104
>
> And i would like to create a monthly withdrawals report as follows:
>
> Machine Day1 Day2 Day3 Day4
> .............................................. Day31 1 11
> 12 13 0 0 2
> 21 22 0 24
> 0 3 31 0 33 0
> 0 4 0 0 0
> 0 0
>
> Can you please help?
>

Doing this purely in SQL is difficult. The reason is because there is no way
to say things like "all the dates in a month, eg, 7/1, 7/2, 7/3, ... 7/31" in
SQL.

What I would do is select the data using aggregates and "group by" the
machine. You can specify how many days or months to aggregate across by
"group by date_trunc("day", Date)" or "date_trunc("month", Date)" or whatever
you like. You can get some interesting behavior by combining that with
date_part() (IE, grouping by day of the week, to compare Mondays with
Tuesdays.)

The data you get back will look something like this:

Machine | Date | Withdrawals
- ---
1 | June 1 | 50
1 | June 7 | 65
1 | June 8 | 12
2 | June 2 | 14
2 | June 9 | 19

When you go to display the code, you populate a list of dates and withdrawals
for each machine. You fill in '0' for the missing dates. Then displaying it
is easy. You can also probably iterate over the results if everything is
sorted, and fill in the missing dates as you encounter them.

- --
Jonathan Gardner <jgardner(at)jonathangardner(dot)net>
(was jgardn(at)alumni(dot)washington(dot)edu)
Live Free, Use Linux!
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