Re: advice on how to store variable attributes

On Oct 22, 2011, at 6:41, Linos <info@linos.es> wrote:

> Hi all,
>    i need a little of advice on what could be the best way to store this information.
>
> We need to calculate the difference in costs for our operations, we are already
> storing our vendor invoices in the database so calculate the monetary change it
> is a no-brainer but we need to store special attributes for any of the invoices
> that we need to compare too, for example:
>    -electric provider: total Kw.
>    -water provider: total m3.
>    -car maintenance: kilometers of the car.
>    -mobile phones provider: international call minutes, national minutes, number
> of sms, etc..
>
> And much more types/variables, the number of variables can change, not every day
> but still can change, i would like that they can be defined/changed from our
> application, so alter table to add columns don't seem the best way (still an
> option though). We will have "generic" reports that will show us changes in
> costs and specific reports for the types with "extended attributes" that we want
> to compare.
>
> To compare values from this "extended attributes" i think we have two ways:
>    1- have them in columns and use standard SQL.
>    2- create the columns with a function that reads this attrs and create the columns.
>
> So far i thin we have this options:
>    1- a bunch of columns that would be null except when the type of the invoice
> uses them.
>    2- a table related with the vendor invoices table for every type of invoice
> with his specifics columns.
>    3- a key/value in a separate table related with the vendor invoices table where
> i store the extended attrs of every invoice that needs them.
>    4- use a hstore column in the vendor invoces table to store this attrs.
>
> The first two have the problem of probably changes to the number of attributes
> of every type and give a more closed solution, apart from that 1- seems to be a
> bit awkward and 2- would need the application that creates the query to know
> with what table should join for every type (other point we will need to change
> if we want to create new invoices types).
>
> The last two have his own problems too, with 3 i will need to create a function
> that return rows as columns to compare them, with 4- given that i will store the
> attrs of every type in the database anyway i can use the operator -> (with a
> CASE using operator ? returning 0 if the searched attr it is not in the hstore)
> but still don't seem a clean solution for me.
>
> For me it seems i am missing something, probably any of you have a much more
> elegant (or correct) way to handle this situation, what would be your advice?
> Thanks.
>
>

Create a table with a single numeric column and multiple category columns.

( amount_value, amount_unit, amount_category, vendor_id )

If necessary each "amount_value" data type should have it's own table since the processing logic will vary (I.e., you
cannotsubtract text or Boolean values). 

You are , in effect, creating multiple tables but combining them into one and using the category column to distinguish
betweenthem. 

David J.